xen/hvc: only notify if we actually sent something

Don't spam dom0/xenconsoled with events unless we've actually added
something to the ring.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

drivers/char/hvc_xen.c

@@ -74,7 +74,8 @@ static int __write_console(const char *data, int len)
 	wmb();			/* write ring before updating pointer */
 	intf->out_prod = prod;
 
-	notify_daemon();
+	if (sent)
+		notify_daemon();
 	return sent;
 }