nbd: set TCP_NODELAY

Disable the Nagle algorithm to reduce latency.  Note this means we must
also use TCP_CORK when sending header followed by payload to avoid
fragmenting lots of little packets.  The previous patch took care of
that.
Suggested-by: NNick Thomas <nick@bytemark.co.uk>
Tested-by: NNick Thomas <nick@bytemark.co.uk>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

block/nbd.c

@@ -406,6 +406,9 @@ static int nbd_establish_connection(BlockDriverState *bs)
         sock = unix_socket_outgoing(qemu_opt_get(s->socket_opts, "path"));
     } else {
         sock = tcp_socket_outgoing_opts(s->socket_opts);
+        if (sock >= 0) {
+            socket_set_nodelay(sock);
+        }
     }
 
     /* Failed to establish connection */