Fix with libxml >=2.14

Index: src/virt.c
--- src/virt.c.orig
+++ src/virt.c
@@ -2205,7 +2205,7 @@ static int domain_lifecycle_event_cb(__attribute__((un
   return 0;
 }
 
-static void virt_eventloop_timeout_cb(int timer ATTRIBUTE_UNUSED,
+static void virt_eventloop_timeout_cb(int timer __attribute__((unused)),
                                       void *timer_info) {}
 
 static int register_event_impl(void) {
