Introduce guest_handle_subrange_okay() for checking sub-sections of an

Introduce guest_handle_subrange_okay() for checking sub-sections of an
argument array. Needed where a compat shim is splitting up a 32-bit
guest's larger argument array, and only the currently-active part of
the translated array is contained within the compat_arg_xlat_area.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>