vfio: fix use-after-free in display

Calling ramfb_display_update() might replace the DisplaySurface with the
boot display, which in turn will free the currently active
DisplaySurface.

So clear our DisplaySurface pinter (dpy->region.surface pointer) to (a)
avoid use-after-free and (b) force replacing the boot display with the
real display when switching back.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Message-id: 20200713124520.23266-1-kraxel@redhat.com

hw/vfio/display.c

@@ -405,6 +405,7 @@ static void vfio_display_region_update(void *opaque)
     if (!plane.drm_format || !plane.size) {
         if (dpy->ramfb) {
             ramfb_display_update(dpy->con, dpy->ramfb);
+            dpy->region.surface = NULL;
         }
         return;
     }