x86/vlapic: allow setting APIC_SPIV_FOCUS_DISABLED in x2APIC mode

x86/vlapic: allow setting APIC_SPIV_FOCUS_DISABLED in x2APIC mode

Current code unconditionally prevents setting APIC_SPIV_FOCUS_DISABLED
regardless of the processor model, which is not correct according to
the specification.

This issue was discovered while trying to boot a pvshim with x2APIC
enabled.

Always allow setting APIC_SPIV_FOCUS_DISABLED: the local APIC
provided to guests is emulated by Xen, and as such doesn't depend on
the features found on the hardware processor. Note for example that
Xen offers x2APIC support to guests even when the underlying hardware
doesn't have such feature.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>