x86/cpuid: Advertise no-lmsl unilaterally to hvm guests

x86/cpuid: Advertise no-lmsl unilaterally to hvm guests

While part of the original AMD64 spec, Long Mode Segment Limit was a feature
not picked up by Intel, and therefore didn't see much adoption in software.
AMD have finally dropped the feature from hardware, and allocated a CPUID bit
to indicate its absence.

Xen has never supported the feature for guests, even when running on capable
hardware, so advertise the feature's absence unilaterally.

There is nothing specifically wrong with exposing this bit to PV guests, but
the PV ABI doesn't include a working concept of MSR_EFER in the first place,
so exposing it to PV guests would be out-of-place.

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