x86/cpuid: Introduce and use default CPUID policies

x86/cpuid: Introduce and use default CPUID policies

For now, the default and max policies remain identical, but this will change
in the future.

Introduce calculate_{pv,hvm}_def_policy().  As *_def derives from *_max, quite
a bit of the derivation logic can be avoided the second time around - this
will cope with simple feature differences for now.

Update XEN_SYSCTL_get_cpu_* and init_domain_cpuid_policy() to use the default
policies as appropriate.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>