It's needed to provide keepalive for nbd client to track server
availability.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190725094937.32454-1-vsementsov@virtuozzo.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Daniel P. Berrangé <berrange@redhat.com>
[eblake: Fix error message typo]
Signed-off-by: Eric Blake <eblake@redhat.com>