Comment 3 for bug 1906266

Ignore c#2, stephenfin correctly points out that the following landed in 6.1.0 relaxing this:

https://github.com/libvirt/libvirt/commit/ae9e6c2a2b75d958995c661f7bb64ed4353a6404

We shouldn't need to rebase existing disks without a backing file format as a result.