qemu: backup: Add partial validation of incremental backup checkpoint
qemu: backup: Add partial validation of incremental backup checkpoint

Verify that the checkpoint requested by an incremental backup exists.
Unfortunately validating whether the checkpoint configuration actually
matches the disk may not be reasonably feasible as the disk may have
been renamed/snapshotted/etc. We still rely on bitmap presence.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt,libvirt/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,jfehlig/libvirt,olafhering/libvirt,crobinso/libvirt,nertpinx/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,jfehlig/libvirt,jfehlig/libvirt,crobinso/libvirt,nertpinx/libvirt,libvirt/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt,nertpinx/libvirt,olafhering/libvirt