vagrant-libvirt post processor: Cannot build vagrant-libvirt box from an another vagrant-libvirt box, unexpected EOF

_This issue was originally opened by @vadorovsky in https://github.com/hashicorp/packer/issues/11148 and has been migrated to this repository. The original issue description is below._

---
#### Overview of the Issue

I'm trying to build my own vagrant-libvirt box based on this one:

https://app.vagrantup.com/opensuse/boxes/Tumbleweed.x86_64

Here is the buildfile I'm using:

https://github.com/rancher-sandbox/enclave/blob/vagrant-workers/contrib/packer/control-plane-libvirt.json

#### Reproduction Steps

Do

```
packer build -force control-plane-libvirt.json
```

on the following buildfile: https://github.com/rancher-sandbox/enclave/blob/vagrant-workers/contrib/packer/control-plane-libvirt.json

### Packer version

Packer v1.7.3

### Simplified Packer Buildfile

https://github.com/rancher-sandbox/enclave/blob/vagrant-workers/contrib/packer/control-plane-libvirt.json

### Operating system and Environment details

openSUSE Tumbleweed
x86_64
kernel 5.13.0-1-default
libvirt 7.5.0

### Log Fragments and crash.log files

(Probably) the most important fragment:

```
2021/07/14 19:16:05 ui: ==> vagrant (vagrant): Creating a dummy Vagrant box to ensure the host system can create one correctly
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: Turning dir into box: /tmp/packer006449265 => /tmp/box-601523484.box
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: Compressing with gzip compression level: -1
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: Skipping directory '/tmp/packer006449265' for box '/tmp/box-601523484.box'
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: Box add: '/tmp/packer006449265/metadata.json' to '/tmp/box-601523484.box'
2021/07/14 19:16:05 ui: ==> vagrant (vagrant): Creating Vagrant box for 'libvirt' provider
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: panic: interface conversion: interface {} is nil, not string
2021/07/14 19:16:05 packer-post-processor-vagrant plugin:
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: goroutine 94 [running]:
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant.(*LibVirtProvider).Process(0x8e563c0, 0x5e66138, 0xc000048090, 0x5e66180, 0xc0000fe080, 0xc0007aa408, 0x14, 0x0, 0x5, 0x100, ...)
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: /home/circleci/go/pkg/mod/github.com/hashicorp/packer-plugin-vagrant@v1.0.0/post-processor/vagrant/libvirt.go:67 +0x918
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant.(*PostProcessor).PostProcessProvider(0xc000e3f1e0, 0xc00053c560, 0x7, 0x5e24c90, 0x8e563c0, 0x5e66138, 0xc000048090, 0x5e66180, 0xc0000fe080, 0x0, ...)
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: /home/circleci/go/pkg/mod/github.com/hashicorp/packer-plugin-vagrant@v1.0.0/post-processor/vagrant/post-processor.go:158 +0x831
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: github.com/hashicorp/packer-plugin-vagrant/post-processor/vagrant.(*PostProcessor).PostProcess(0xc000e3f1e0, 0x5e4ce20, 0xc000278100, 0x5e66138, 0xc000048090, 0x5e66180, 0xc0000fe080, 0x5e27cb0, 0x203000, 0x5dbc5c0, ...)
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: /home/circleci/go/pkg/mod/github.com/hashicorp/packer-plugin-vagrant@v1.0.0/post-processor/vagrant/post-processor.go:230 +0x199
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: github.com/hashicorp/packer-plugin-sdk/rpc.(*PostProcessorServer).PostProcess(0xc00011e040, 0x1, 0xc00041a000, 0x0, 0x0)
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: /home/circleci/go/pkg/mod/github.com/hashicorp/packer-plugin-sdk@v0.2.3/rpc/post_processor.go:108 +0x19a
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: reflect.Value.call(0xc000100480, 0xc00000e3a8, 0x13, 0x56cc20d, 0x4, 0xc000041f08, 0x3, 0x3, 0xc00021e140, 0xc000219f68, ...)
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: /usr/local/go/src/reflect/value.go:476 +0x8e7
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: reflect.Value.Call(0xc000100480, 0xc00000e3a8, 0x13, 0xc000219f08, 0x3, 0x3, 0xc00021e140, 0x4aaf820, 0xc0005ae000)
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: /usr/local/go/src/reflect/value.go:337 +0xb9
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: net/rpc.(*service).call(0xc00053e2c0, 0xc0004fe190, 0xc0005ae310, 0xc0005ae320, 0xc00079a200, 0xc00047c160, 0x469d880, 0xc0005ae00c, 0x18a, 0x44d48a0, ...)
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: /usr/local/go/src/net/rpc/server.go:377 +0x189
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: created by net/rpc.(*Server).ServeCodec
2021/07/14 19:16:05 packer-post-processor-vagrant plugin: /usr/local/go/src/net/rpc/server.go:474 +0x44d
2021/07/14 19:16:05 [INFO] (telemetry) ending vagrant
2021/07/14 19:16:05 Deleting original artifact for build 'vagrant'
2021/07/14 19:16:05 /home/vadorovsky/bin/packer: plugin process exited
2021/07/14 19:16:05 ui error: Build 'vagrant' errored after 3 minutes 197 milliseconds: 1 error(s) occurred:
```

crash.log:
https://gist.github.com/vadorovsky/797599ab4c3df4ebce6514a274dd5b6f

Start in post-processor/vagrant/libvirt.go at LibVirtProvider.Process, especially the location reported at line 67, and reproduce with the linked control-plane-libvirt.json using `packer build -force control-plane-libvirt.json`. Done means building a Vagrant box from another vagrant-libvirt box no longer panics with an interface conversion error or unexpected EOF.

Written by the indexing model from the issue text.