sandbox: Implement host dev [device]

A common pattern to check if a certain device exists (e.g. in
config_distro_bootcmd) is to use: <interface> dev [device]

Implement host dev [device] so this pattern can be used for sandbox host
devices.

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