libs/guest: introduce helper to fetch a system cpu policy

libs/guest: introduce helper to fetch a system cpu policy

Such helper is based on the existing functions to fetch a CPUID and
MSR policies, but uses the xc_cpu_policy_t type to return the data to
the caller.

Note some helper functions are introduced, those are split from
xc_cpu_policy_get_system because they will be used by other functions
also.

No user of the interface introduced on the patch.

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