x86/hvm: always expose x2APIC feature in max HVM cpuid policy

x86/hvm: always expose x2APIC feature in max HVM cpuid policy

On hardware without x2APIC support Xen emulated local APIC will
provide such mode, and hence the feature should be set in the maximum
HVM cpuid policy.

Not exposing it in the maximum policy results in HVM domains not
getting such feature exposed unless it's also supported by the
underlying hardware.

This was regressed by c/s 3e0c8272f20 which caused x2APIC not to be enabled
unilaterally for HVM guests.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>