Add support for `provider_override = "qemu"`

I am using the [vagrant-qemu](https://github.com/ppggff/vagrant-qemu) plugin to run QEMU boxes on macOS.

I'd like the ability to generate Vagrant boxes for this provider, such as the one below, but at present, `qemu` is not one of the allowable values for providers.

``` hcl
source "file" "fedora-coreos-qemu-aarch64" {
source = "../../artifacts/fcos-images/fedora-coreos-qemu.aarch64.qcow2"
target = "fedora-coreos-qemu.aarch64.qcow2"
}

build {
sources = ["file.fedora-coreos-qemu-aarch64"]

post-processors {
post-processor "vagrant" {
vagrantfile_template = "./qemu/Vagrantfile"
include = ["fedora-coreos-qemu.aarch64.qcow2"]
keep_input_artifact = false
provider_override = "qemu"
}
}
}
```

This fails on packer 1.9.2 with the following message:

```
Error: Failed preparing post-processor-block "vagrant" ""

on packer.pkr.hcl line 10:
(source code not available)

The given provider_override qemu is not valid. Please choose from one of docker,
digitalocean, azure, scaleway, libvirt, hyperv, parallels, google, lxc,
virtualbox, aws, vmware
```