xen/acpi: revert pad config check in xen_check_mwait

xen/acpi: revert pad config check in xen_check_mwait

With Xen acpi pad logic added into kernel, we can now revert xen mwait related
patch 8153dfdd5dbfa1c2f0a8f3de39333b51c00e137c ("xen/enlighten: Disable
MWAIT_LEAF so that acpi-pad won't be loaded. "). The reason is, when running under
newer Xen platform, Xen pad driver would be early loaded, so native pad driver
would fail to be loaded, and hence no mwait/monitor #UD risk again.

Another point is, only Xen4.2 or later support Xen acpi pad, so we won't expose
mwait cpuid capability when running under older Xen platform.

Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>