Commit d75f865f caused a job-deadlock if
a VM is running the backup job and being destroyed as it removed the
cleanup of the async job type and there was nothing to clean up the
backup job.

Add an explicit cleanup of the backup job when destroying a VM.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>