block/nbd-client: assert qiov len once in nbd_co_request

Also improve the assertion: check that qiov is NULL for other commands
than CMD_READ and CMD_WRITE.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20171012095319.136610-2-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
