net: use socket_set_nodelay() for -netdev socket

Reduce -netdev socket latency by disabling the Nagle algorithm on
SOCK_STREAM sockets in net/socket.c.  Since we are tunelling Ethernet
over TCP we shouldn't artificially delay outgoing packets, let the guest
decide packet scheduling.

I already get sub-millisecond -netdev socket ping times on localhost, so
there was no measurable difference in my testing.  This won't hurt
though and may improve remote socket performance.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Reviewed-by: NDaniel P. Berrange <berrange@redhat.com>

net/socket.c

@@ -438,6 +438,9 @@ static NetSocketState *net_socket_fd_init_stream(NetClientState *peer,
     s->fd = fd;
     s->listen_fd = -1;
 
+    /* Disable Nagle algorithm on TCP sockets to reduce latency */
+    socket_set_nodelay(fd);
+
     if (is_connected) {
         net_socket_connect(s);
     } else {