sandbox: Provide a way to bind fixed/removeable devices

sandbox: Provide a way to bind fixed/removeable devices

At present when a file is bound to a host device it is always marked as
removeable. Arguably the device is removeable, since it can be unbound at
will. However while it is bound, it is not considered removable by the
user. Also it is useful to be able to model both fixed and removeable
devices for code that distinguishes them.

Add a -r flag to the 'host bind' command and plumb it through to provide
this feature.

Signed-off-by: Simon Glass <sjg@chromium.org>