Compact backing chain (sparsify + compress)

[Christopher Pereira <kripper@imatronix.cl>]

Hi,

I would like to revisit this old thread from 2016 with a special use 
case that I believe should be a standard `virsh` command:
https://lists.gnu.org/archive/html/qemu-devel/2016-12/msg03571.html

**Summary:**

Given this QEMU backing chain:
`base <- snap1 <- snap2 <- snap3 (active)`

We want to merge `base <- snap1 <- snap2` into a new snapshot 
`collapsed-base` that is:
1. Sparsified (`virt-sparsify`)
2. Compressed

The resulting backing chain would be:
`collapsed-base <- snap3 (active)`

**Motivation:**

- We perform daily backup snapshots and never modify existing files (too 
dangerous). We only rebase.
- We collapse older chains into a new `collapsed-base` snapshot to limit 
chain size and avoid performance degradation.

We have been doing this successfully for over 10 years using:

- `qemu-img convert`
- `virt-sparsify`
- `virsh save`
- `qemu-img rebase`
- `virsh resume`

**Problems:**

- There is a small downtime due to `virsh save`/`resume`.
- In recent QEMU versions, `virsh` adds a `backingStore` tag to the XML 
even when using the `--no-metadata` option. This causes inconsistencies 
after `qemu-img rebase`.

We didn’t use QMP because it didn’t support sparsify + compression in 
the past.

**Questions:**

- Is there now a better way to achieve this?
- Could this feature be implemented or supported directly in `virsh`?

In my opinion, this would be the ideal backup solution: we could travel 
in time, sync immutable snapshots to a remote backup server, and 
maintain performance.