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: Li Qiang <liqiang6-s@360.cn>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 5884626f.5b2f6b0a.1bfff.3037@mx.google.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
