qapi event: convert POWERDOWN

There is no existing comments for POWERDOWN in doc/qmp/qmp-events.txt,
so no change on it like other conversion patch.

Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/vl.c b/vl.c
index 1d82b12..abb56d3 100644
--- a/vl.c
+++ b/vl.c
@@ -1932,7 +1932,7 @@
 
 static void qemu_system_powerdown(void)
 {
-    monitor_protocol_event(QEVENT_POWERDOWN, NULL);
+    qapi_event_send_powerdown(&error_abort);
     notifier_list_notify(&powerdown_notifiers, NULL);
 }