x86/domctl: Update PV domain cpumasks when setting cpuid policy

x86/domctl: Update PV domain cpumasks when setting cpuid policy

This allows PV domains with different featuresets to observe different values
from a native cpuid instruction, on supporting hardware.

It is important to leak the host view of X2APIC, HTT and CMP_LEGACY through to
guests, even though they could be hidden.  These flags affect how to interpret
other cpuid leaves which are not maskable.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>