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>

drivers/acpi/acpi_pad.c

@@ -26,6 +26,7 @@
 #include <linux/slab.h>
 #include <linux/acpi.h>
 #include <asm/mwait.h>
+#include <xen/xen.h>
 
 #define ACPI_PROCESSOR_AGGREGATOR_CLASS	"acpi_pad"
 #define ACPI_PROCESSOR_AGGREGATOR_DEVICE_NAME "Processor Aggregator"
@@ -477,6 +478,10 @@ static struct acpi_driver acpi_pad_driver = {
 
 static int __init acpi_pad_init(void)
 {
+	/* Xen ACPI PAD is used when running as Xen Dom0. */
+	if (xen_initial_domain())
+		return -ENODEV;
+
 	power_saving_mwait_init();
 	if (power_saving_mwait_eax == 0)
 		return -EINVAL;