Add missing linefeed in error message

The error message for an unknown network device given to
monitor command set_link looks better with a terminating
linefeed.
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

net.c

@@ -2917,7 +2917,7 @@ void do_set_link(Monitor *mon, const char *name, const char *up_or_down)
 done:
 
     if (!vc) {
-        monitor_printf(mon, "could not find network device '%s'", name);
+        monitor_printf(mon, "could not find network device '%s'\n", name);
         return;
     }