"Post-processor failed: unexpected EOF" vagrant post processor on libvirt image

### Overview of the Issue

I get an error "Post-processor failed: unexpected EOF" when trying to build a box from an existing libvirt kvm image.
I prebuild a kvm image with the qemu provider.
I tried to convert it with the vagrant post-processor but it fails. I have no problem building an image in one step but it doesn't work in two steps.
I used the example from the documentation using Artifice post-processor and overrode the provider.

### Plugin and Packer version

I used :
- Packer **v1.10.1**
- packer-plugin-qemu **v1.0.10**
- packer-plugin-vagrant **v1.1.2**

### Packer Buildfile

I used the following configuration :

```
packer {
required_plugins {
vagrant = {
version = ">= 1.1.1"
source = "github.com/hashicorp/vagrant"
}
}
}

source "null" "fedora" {
communicator = "none"
}

build {
sources = [
"source.null.fedora"
]

post-processors {
post-processor "artifice" {
files = [
"packer_images/fedora_workstation.qcow2"
]
}

post-processor "vagrant" {
keep_input_artifact = true
provider_override = "libvirt"
}
}
}
```

### Operating system and Environment details

I build on Fedora 38

### Logs

null.fedora: output will be in this color.

==> null.fedora: Running post-processor: (type artifice)
==> null.fedora (artifice): Using these artifact files: packer_images/fedora_workstation.qcow2
==> null.fedora: Running post-processor: (type vagrant)
==> null.fedora (vagrant): Creating a dummy Vagrant box to ensure the host system can create one correctly
==> null.fedora (vagrant): Creating Vagrant box for 'libvirt' provider
Build 'null.fedora' errored after 42 milliseconds 317 microseconds: 1 error(s) occurred:

* Post-processor failed: unexpected EOF

==> Wait completed after 42 milliseconds 507 microseconds

==> Some builds didn't complete successfully and had errors:
--> null.fedora: 1 error(s) occurred:

* Post-processor failed: unexpected EOF

==> Builds finished but no artifacts were created.
panic: interface conversion: interface {} is nil, not string
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin:
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: goroutine 9 [running]:
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant.(*LibVirtProvider).Process(0x11215ee?, {0x1447508, 0xc0007be6f0}, {0x1447550, 0xc0006c2720}, {0xc0006a8000, 0x15})
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant/libvirt.go:71 +0x736
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant.(*PostProcessor).PostProcessProvider(0xc0006aa968?, {0xc0006aa968, 0x7}, {0x1441010, 0x1c6d1c8}, {0x1447508, 0xc0007be6f0}, {0x1447550, 0xc0006c2720})
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant/post-processor.go:176 +0xa7a
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant.(*PostProcessor).PostProcess(0xc0001b38c0, {0xc00019c010?, 0x0?}, {0x1447508, 0xc0007be6f0}, {0x1447550?, 0xc0006c2720?})
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant/post-processor.go:251 +0x154
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: github.com/hashicorp/packer-plugin-sdk/rpc.(*PostProcessorServer).PostProcess(0xc0003ca100, 0x19cdc8?, 0xc00079a990)
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: github.com/hashicorp/packer-plugin-sdk@v0.5.2/rpc/post_processor.go:111 +0x197
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: reflect.Value.call({0xc0001a0a80?, 0xc00019e728?, 0x13?}, {0x1121686, 0x4}, {0xc0000b6ef8, 0x3, 0x3?})
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: reflect/value.go:586 +0xb07
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: reflect.Value.Call({0xc0001a0a80?, 0xc00019e728?, 0x8000000000000000?}, {0xc0002076f8?, 0x9?, 0x0?})
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: reflect/value.go:370 +0xbc
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: net/rpc.(*service).call(0xc0003ca140, 0x1128408?, 0x9?, 0xc00019c0a0, 0xc0006b0300, 0x9?, {0xf55f80?, 0xc00019cdb8?, 0x0?}, {0xf23e60, ...}, ...)
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: net/rpc/server.go:382 +0x226
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: created by net/rpc.(*Server).ServeCodec
2024/03/01 10:15:03 packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64 plugin: net/rpc/server.go:479 +0x3fe
2024/03/01 10:15:03 /home/jorge/.config/packer/plugins/github.com/hashicorp/vagrant/packer-plugin-vagrant_v1.1.2_x5.0_linux_amd64: plugin process exited
2024/03/01 10:15:03 [INFO] (telemetry) ending vagrant
2024/03/01 10:15:03 ui error: Build 'null.fedora' errored after 42 milliseconds 317 microseconds: 1 error(s) occurred:

* Post-processor failed: unexpected EOF
2024/03/01 10:15:03 ui:
==> Wait completed after 42 milliseconds 507 microseconds
2024/03/01 10:15:03 machine readable: error-count []string{"1"}
2024/03/01 10:15:03 ui error:
==> Some builds didn't complete successfully and had errors:
2024/03/01 10:15:03 machine readable: null.fedora,error []string{"1 error(s) occurred:\n\n* Post-processor failed: unexpected EOF"}
2024/03/01 10:15:03 ui error: --> null.fedora: 1 error(s) occurred:

* Post-processor failed: unexpected EOF
2024/03/01 10:15:03 ui:
==> Builds finished but no artifacts were created.
2024/03/01 10:15:03 [INFO] (telemetry) Finalizing.
2024/03/01 10:15:03 waiting for all plugin processes to complete...
2024/03/01 10:15:03 /usr/bin/packer: plugin process exited
2024/03/01 10:15:03 /usr/bin/packer: plugin process exited

!!!!!!!!!!!!!!!!!!!!!!!!!!! PACKER CRASH !!!!!!!!!!!!!!!!!!!!!!!!!!!!

Packer crashed! This is always indicative of a bug within Packer.
A crash log has been placed at "crash.log" relative to your current
working directory. It would be immensely helpful if you could please
report the crash with Packer[1] so that we can fix this.

[1]: https://github.com/hashicorp/packer/issues

!!!!!!!!!!!!!!!!!!!!!!!!!!! PACKER CRASH !!!!!!!!!!!!!!!!!!!!!!!!!!!!

Reproduce the two-step Artifice-to-Vagrant build from the configuration, then inspect post-processor/vagrant/libvirt.go:71 and the surrounding provider path using the supplied panic. Done means the libvirt artifact is handled without the unexpected EOF or plugin panic, with either a successful Vagrant box or a useful error.

Written by the indexing model from the issue text.