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

When running as pv domain xen_cpuid() is being used instead of
native_cpuid(). In xen_cpuid() the acc feature (thermal monitoring)
is indicated as not being present by special casing the related
cpuid leaf.

Instead of delivering fake cpuid values clear the cpu capability bit
for acc 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>