x86: calculate maximum host and guest featuresets

x86: calculate maximum host and guest featuresets

All of this information will be used by the toolstack to make informed
levelling decisions for VMs, and by Xen to sanity check toolstack-provided
information.

The split between the shadow and hap HVM masks is necessary due to the lack of
a "get cpuid policy" hypercall.  Multi-host toolstacks (i.e. not libxl)
dealing with hap and non-hap capable hosts need to be able to calculate that
migrating a shadow guest is safe.

Future planned development work will implement proper cpuid policy handing in
Xen, including a "get policy" hypercall, but until then, the difference is
made available for toolstack use via a non-stable interface.

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