x86/cpuid: Split update_domain_cpuid_info() in half

x86/cpuid: Split update_domain_cpuid_info() in half

update_domain_cpuid_info() currently serves two purposes.  First to merge new
CPUID data from the toolstack, and second, to perform any necessary updating
of derived domain/vcpu settings.

The first part of this is going to be superseded by a new and substantially
more efficient hypercall.

Carve the second part out into a new domain_cpu_policy_changed() helper, and
call this from the remains of update_domain_cpuid_info().

This does drop the call_policy_changed, but with the new hypercall in place,
the common case will be a single call per domain.  Dropping the optimisation
here allows for a cleaner set of following changes.

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