sandbox: only do sandboxfs for hostfs interface

Only do sandbox filesystem access when using the hostfs device
interface, rather then falling back to it in all cases. This prevents
confusion situations due to the fallback being taken rather then an
unsupported error being raised.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>