vl: add runstate_set tracepoint

This patch enables us to know RunState transition. It will be userful
for investigation when the trouble occured in special event such like
live migration, shutdown, suspend, and so on.

Signed-off-by: Kazuya Saito <saito.kazuya@jp.fujitsu.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/vl.c b/vl.c
index 7643f16..770cb7f 100644
--- a/vl.c
+++ b/vl.c
@@ -642,7 +642,7 @@
                 RunState_lookup[new_state]);
         abort();
     }
-
+    trace_runstate_set(new_state);
     current_run_state = new_state;
 }