main: allocate gui_timer only once.

fix memory leak.
there is no need to allocate more than one gui_timer.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

vl.c

@@ -3794,6 +3794,7 @@ int main(int argc, char **argv, char **envp)
         if (dcl->dpy_refresh != NULL) {
             ds->gui_timer = qemu_new_timer(rt_clock, gui_update, ds);
             qemu_mod_timer(ds->gui_timer, qemu_get_clock(rt_clock));
+            break;
         }
         dcl = dcl->next;
     }