Check shutdown_requested timely.

After a recent merge of the ioemu-remote tree several weeks ago, I find in main() -> termsig_seup(), ioemu sets a SIGHUP handler that sets a global variable shutdown_requested to 1, but ioemu doesn't check the variable and as a result, ioemu wouldn't terminate when xend sends a SIGHUP to it; finally xend has to send a SIGKILL to it in 10 seconds. The small patch adds a check to terminate ioemu timely.     

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>

diff --git a/vl.c b/vl.c
index 99ebd9d..607e2c4 100644
--- a/vl.c
+++ b/vl.c
@@ -8052,6 +8052,10 @@ void main_loop_wait(int timeout)
                 pioh = &ioh->next;
         }
     }
+    else {
+        if (qemu_shutdown_requested())
+            exit(0);
+    }
 #if defined(CONFIG_SLIRP)
     if (slirp_inited) {
         if (ret < 0) {