x86/cpuid: Handle leaf 0xb in guest_cpuid()

x86/cpuid: Handle leaf 0xb in guest_cpuid()

Leaf 0xb is reserved by AMD, and uniformly hidden from guests by the toolstack
logic and hypervisor PV logic.  The previous dynamic logic filled in the
x2APIC ID for all HVM guests.

In practice, leaf 0xb is tightly linked with x2APIC, and x2APIC is offered to
guests on AMD hardware, as Xen's APIC emulation is x2APIC capable even if
hardware isn't.

Sensibly exposing the rest of the leaf requires further topology
infrastructure.

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