Only shutdown video subsytem in sdl_cleanup

Depending on the order in which atexit handlers are called SDL might
try to join on an audio thread without said thread ever being notified
that it must stop, hence QEMU will forever block in pthread_join call.
Signed-off-by: Nmalc <av1474@comtv.ru>

sdl.c

@@ -753,7 +753,7 @@ static void sdl_cleanup(void)
 {
     if (guest_sprite)
         SDL_FreeCursor(guest_sprite);
-    SDL_Quit();
+    SDL_QuitSubSystem(SDL_INIT_VIDEO);
 }
 
 void sdl_display_init(DisplayState *ds, int full_screen, int no_frame)