If the guest trigger following sequences, the attach_backing will be leaked:

	vg_resource_create_2d
	vg_resource_attach_backing
	vg_resource_unref

This patch fix this by freeing 'res->iov' in vg_resource_destroy.

Fixes: CVE-2021-3544
Reported-by: Li Qiang <liq3ea@163.com>
virtio-gpu fix: 5e8e3c4c ("virtio-gpu: fix resource leak
in virgl_cmd_resource_unref")

Reviewed-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Li Qiang <liq3ea@163.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210516030403.107723-5-liq3ea@163.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>