It will be used to stop retrying NBD requests on mirror cancel.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210205163720.887197-2-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>