x86/domctl: Move all CPUID update logic into update_domain_cpuid_info()

x86/domctl: Move all CPUID update logic into update_domain_cpuid_info()

This simplifies the XEN_DOMCTL_set_cpuid handling, splitting the safety logic
away from the internals of how an update is completed.

The legacy cpuids[] logic is left in alone in a fuction, as it wont survive
very long.  update_domain_cpuid_info() gains a small performance optimisation
to skip all update activites for leaves which won't have any impact on the
guest.  This is temporary until the new hypercall API is completed.

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