virtio-gpu: fix memory leak in set scanout

In virtio_gpu_set_scanout function, when creating the 'rect'
its refcount is set to 2, by pixman_image_create_bits and
qemu_create_displaysurface_pixman function. This can lead
a memory leak issues. This patch avoid this issue.
Signed-off-by: NLi Qiang <liqiang6-s@360.cn>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 5884626f.5b2f6b0a.1bfff.3037@mx.google.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/display/virtio-gpu.c

@@ -608,6 +608,7 @@ static void virtio_gpu_set_scanout(VirtIOGPU *g,
             cmd->error = VIRTIO_GPU_RESP_ERR_UNSPEC;
             return;
         }
+        pixman_image_unref(rect);
         dpy_gfx_replace_surface(g->scanout[ss.scanout_id].con, scanout->ds);
     }