mirror: block all operations on the target image during the job

There's nothing preventing the target image from being used by other
operations during the 'drive-mirror' job, so we should block them all
until the job is done.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 82b88fd04cde918a08a6f9a4ab85626d7fd7b502.1446475331.git.berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
