x86/cpuid: Avoid unconditionally clobbering ITSC for guests

x86/cpuid: Avoid unconditionally clobbering ITSC for guests

In general, Invariant TSC is not a feature which can be advertised to guests,
because it cannot be guaranteed across migrate.  domain_cpuid() goes so far as
to deliberately clobber the feature flag under a number of circumstances.

Because ITSC is absent from the static {pv,hvm}_featureset masks, c/s b648feff
"xen/x86: Improvements to in-hypervisor cpuid sanity checks" caused ITSC to be
unconditionally masked out.

As an interim solution, include the hosts idea of ITSC along with the static
{pv,hvm}_featureset when restricting the guests view of features.  This causes
the hardware domain, and VMs explicitly configured with ITSC and no-migrate to
be offered ITSC (subject to hardware availability).

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <JBeulich@suse.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>