virtio-gpu: fix memory leak in virtio_gpu_resource_create_2d

In virtio gpu resource create dispatch, if the pixman format is zero
it doesn't free the resource object allocated previously. Thus leading
a host memory leak issue. This patch avoid this.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 57df486e.8379240a.c3620.ff81@mx.google.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
