virtio-gpu-3d: fix memory leak in resource attach backing

If the virgl_renderer_resource_attach_iov function fails the
'res_iovs' will be leaked. Add check of the return value to
free the 'res_iovs' when failing.

Signed-off-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1482999086-59795-1-git-send-email-liq3ea@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
