sandbox: block driver using host file/device as backing store

A simple "host" block driver using any host file/device
as backing store.

Adds two sb subcommands for managing host device bindings:
sb bind <dev> [<filename>] - bind "host" device to file
sb info [<dev>]            - show device binding & info

Signed-off-by: Henrik Nordstrom <henrik@henriknordstrom.net>