[feature] Allow use ova in qemu builder

This issue was originally opened by @yamatoRT as hashicorp/packer#9906. It was migrated here as a result of the Packer plugin split. The original body of the issue is below.

Description

I would like import a ova in qemu builder for create vagrant box. The ova may have many disks.
Moreover, I would like select the order of disks because the disk in ova have special name.

Potential configuration

Suggestion 1 :
Add new builder with ova_url

{
"builders": [
    {
      "type": "qemu",
      "ova_url": "<ova_path>",
      "ova_checksum": "<ova_checksum>",
      "disks_order" : [ "disk_1.qcow", "disk_2.qcows"],
      "use_backing_file":true,
      "format": "qcow2",
      "accelerator": "kvm",
      "ssh_username": "root",
      "ssh_password": "root",
      "ssh_timeout": "5m",
      "net_device": "e1000",
      "disk_interface": "ide",
      "boot_wait": "10s"
    }
  ]

}

Suggestion 2 :
Use iso_url and add new option ova

{
"builders": [
    {
      "type": "qemu",
      "iso_url": "<ova_path>",
      "iso_checksum": "<ova_checksum>",
      "ova" : true,
      "disks_order" : [ "disk_1.qcow", "disk_2.qcows"],
      "use_backing_file":true,
      "format": "qcow2",
      "accelerator": "kvm",
      "ssh_username": "root",
      "ssh_password": "root",
      "ssh_timeout": "5m",
      "net_device": "e1000",
      "disk_interface": "ide",
      "boot_wait": "10s"
    }
  ]

}

Start with the QEMU builder configuration and its existing iso_url handling, then review how disk inputs and ordering are represented. Compare the two proposed OVA configuration approaches and determine the expected behavior for multi-disk OVA files, checksums, and disk order. Done means the chosen configuration is documented and validated for creating a Vagrant box.