x86/xen: use capabilities instead of fake cpuid values for acpi

When running as pv domain xen_cpuid() is being used instead of
native_cpuid(). In xen_cpuid() the acpi feature is indicated as not
being present by special casing the related cpuid leaf in case we
are not the initial domain.

Instead of delivering fake cpuid values clear the cpu capability bit
for acpi instead.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>