block/nbd-client: get rid of ssize_t

Use int variable for nbd_co_send_request return value (as
nbd_co_send_request returns int).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20170804151440.320927-6-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>