Libvirt introduced a 'manual' disk snapshot mode which  keeps the VM paused at the point in time where the snapshot was taken (when memory snapshot is used) and allows storage managed externally to be snapshotted at that time.

The 'manual' disk snapshot mode needs to be improved to:

 - ensure that disk metadata (e.g. qcow2 headers) is in good state even when no memory snapshot is done

 - allow 'manual' snapshot for all disks (currently fails if all disks are snapshotted manually)