libs/guest: Don't hide the indirection on xc_cpu_policy_t

libs/guest: Don't hide the indirection on xc_cpu_policy_t

It is bad form in C, perhaps best demonstrated by trying to read
xc_cpu_policy_destroy(), and causes const qualification to have
less-than-obvious behaviour (the hidden pointer becomes const, not the thing
it points at).

xc_cpu_policy_set_domain() needs to drop its (now normal) const qualification,
as the policy object is modified by the serialisation operation.

This also shows up a problem with the x86_cpu_policies_are_compatible(), where
the intermediate pointers are non-const.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>