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.

首先阅读所引用的 libvirt 10.10 data-file 功能说明，并跟踪 CloudStack 如何处理 Ceph 主存储上的 QCOW2 镜像。确定涉及的 storage 和 hypervisor 入口点以及现有测试；完成的标准是，较新的主机能够为基于 Ceph 的 VM 使用 QCOW2 data-file 镜像，同时不破坏现有的镜像处理。

由索引模型根据 Issue 内容生成。