Monitor: Return before exiting with 'quit'

The 'quit' Monitor command (implemented by do_quit()) calls
exit() directly, this is problematic under QMP because QEMU
exits before having a chance to send the ok response.

Clients don't know if QEMU exited because of a problem or
because the 'quit' command has been executed.

This commit fixes that by moving the exit() call to the main
loop, so that do_quit() requests the system to quit, instead
of calling exit() directly.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

monitor.c

@@ -1017,7 +1017,8 @@ static void do_info_cpu_stats(Monitor *mon)
  */
 static int do_quit(Monitor *mon, const QDict *qdict, QObject **ret_data)
 {
-    exit(0);
+    monitor_suspend(mon);
+    qemu_system_exit_request();
     return 0;
 }