xen/x86: Use do_kernel_power_off()

Kernel now supports chained power-off handlers. Use do_kernel_power_off()
that invokes chained power-off handlers. It also invokes legacy
pm_power_off() for now, which will be removed once all drivers will
be converted to the new sys-off API.
Acked-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

arch/x86/xen/enlighten_pv.c

@@ -31,6 +31,7 @@
 #include <linux/gfp.h>
 #include <linux/edd.h>
 #include <linux/objtool.h>
+#include <linux/reboot.h>
 
 #include <xen/xen.h>
 #include <xen/events.h>
@@ -1071,8 +1072,7 @@ static void xen_machine_halt(void)
 
 static void xen_machine_power_off(void)
 {
-	if (pm_power_off)
-		pm_power_off();
+	do_kernel_power_off();
 	xen_reboot(SHUTDOWN_poweroff);
 }