vl: change runstate only if new state is different from current state

Previously, qemu will abort at following scenario:
(qemu) stop
(qemu) system_reset
(qemu) system_reset
(qemu) 2016-04-13T20:54:38.979158Z qemu-system-x86_64: invalid runstate transition: 'prelaunch' -> 'prelaunch'
Signed-off-by: NLi Zhijian <lizhijian@cn.fujitsu.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <1460604352-18630-1-git-send-email-lizhijian@cn.fujitsu.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

vl.c

@@ -690,6 +690,10 @@ void runstate_set(RunState new_state)
 {
     assert(new_state < RUN_STATE__MAX);
 
+    if (current_run_state == new_state) {
+        return;
+    }
+
     if (!runstate_valid_transitions[current_run_state][new_state]) {
         error_report("invalid runstate transition: '%s' -> '%s'",
                      RunState_lookup[current_run_state],