Support many files in Vagrant box for libvirt

[AirTruc]

Hello,

I try to implement this issue hashicorp/packer#10627.
Now, we use this format for metadata.json :

{
  "disks": [
    {
      "path": "box_0.img",
      "format": "qcow2"
    },
    {
      "path": "box_1.img",
      "format": "qcow2"
    },
    {
      "path": "box_2.img",
      "format": "qcow2"
    }
  ],
  "provider": "libvirt"
}

I append new tests.

Closes #10627

[hashicorp-cla]

All committers have signed the CLA.

[nywilken]

@LeConTesteur @electrofelix apologies for the delayed review on this PR. I'm slating it for review so that a member of the team can follow up with the original issue and requested changes. Thanks again for pushing this plugin forward.

[lbajolet-hashicorp]

Hi @LeConTesteur,

As I mentioned in the related issue, sorry for the delay in the review, and thanks for submitting this PR.

Given what we know about the qemu builder, which is the only one producing artifacts that will be processed by this code, there's currently only one image in the output, so we can rely on this to include only the image itself, and not the potential extra efivars.fd raw blob in the volume list.

Additionally, since we've taken so long to merge this, the code conflicts with a change made a while back on the filename check, so that'll have to be fixed before we can merge this PR.

Please let me know if you're still interested to work on this PR, I'll happily review the changes once you're ready.
Otherwise, please let me know and I can continue working on supporting this change.

[lbajolet-hashicorp]

Hi @LeConTesteur,

Thanks for the reroll, with the discussion ongoing in the related issue, I'm more confident this code will work as intended, I just left a few suggestions/nits, but overall this looks good to me.

[lbajolet-hashicorp]

All good to me now @LeConTesteur, thanks for rerolling this.

I'll merge the PR today