qemu_process: abort snapshot delete when daemon starts

qemu_process: abort snapshot delete when daemon starts

If the daemon crashes or is restarted while the snapshot delete is in
progress we have to handle it gracefully to not leave any block jobs
active.

For now we will simply abort the snapshot delete operation so user can
start it again. We need to refuse deleting external snapshots if there
is already another active job as we would have to figure out which jobs
we can abort.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>