Add support for provider_override = "qemu"

[MisterWS]

I am using the 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.

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