Support qcow data-file for Ceph storage

### The required feature described as a wish

As a User/Admin/Operator I would like to have support for the `data-file` functionality of libvirt 10.10 supported on newer hosts. This way one would be able to use a lot of different backup solutions such as virtnbdbackup to back up VMs running on a Ceph primary storage.

https://www.mail-archive.com/devel@lists.libvirt.org/msg07367.html

> * qemu: Support for the 'data-file' QCOW2 image feature
>
> The QEMU hypervisor driver now supports QCOW2 images with 'data-file'
feature present (both when probing form the image itself and when specified
explicitly via ```` element). This can be useful when it's
required to keep data "raw" on disk, but the use case requires features
of the QCOW2 format such as incremental backups.

There are probably a lot of things to take into account, but it would still be a great improvement.

まず、参照されている libvirt 10.10 の data-file 機能の説明を読み、CloudStack が Ceph プライマリストレージ上の QCOW2 イメージをどのように処理しているかを追跡します。関係する storage と hypervisor のエントリーポイント、および既存のテストを特定します。完了の条件は、新しいホストが既存のイメージ処理を壊すことなく、Ceph-backed VM で QCOW2 data-file イメージを使用できることです。

索引モデルが issue の本文から書いたものです。