ACPI / PAD: don't register acpi_pad driver if running as Xen dom0

When running as Xen dom0 a special processor_aggregator driver is
needed.  Don't register the standard driver in this case.

Without that check an error message:

"Error: Driver 'processor_aggregator' is already registered,
aborting..."

will be displayed.

Signed-off-by: Juergen Gross <jgross@suse.com>
[ rjw: Minor fixups ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>