net: EAGAIN handling for net/socket.c UDP

Implement asynchronous send for UDP (or other SOCK_DGRAM) sockets.  If
send fails with EAGAIN we wait for the socket to become writable again.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

net/socket.c

diff --git a/net/socket.c b/net/socket.c
index 7bff536..aabf0a4 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -102,9 +102,19 @@
 static ssize_t net_socket_receive_dgram(NetClientState *nc, const uint8_t *buf, size_t size)
 {
     NetSocketState *s = DO_UPCAST(NetSocketState, nc, nc);
+    ssize_t ret;
 
-    return sendto(s->fd, (const void *)buf, size, 0,
-                  (struct sockaddr *)&s->dgram_dst, sizeof(s->dgram_dst));
+    do {
+        ret = sendto(s->fd, buf, size, 0,
+                     (struct sockaddr *)&s->dgram_dst,
+                     sizeof(s->dgram_dst));
+    } while (ret == -1 && errno == EINTR);
+
+    if (ret == -1 && errno == EAGAIN) {
+        net_socket_write_poll(s, true);
+        return 0;
+    }
+    return ret;
 }
 
 static void net_socket_send(void *opaque)