suspend, xen: enable PM_SLEEP for CONFIG_XEN

Xen save/restore requires PM_SLEEP to be set without requiring
SUSPEND or HIBERNATION.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/power/Kconfig

@@ -82,7 +82,7 @@ config PM_SLEEP_SMP
 
 config PM_SLEEP
 	bool
-	depends on SUSPEND || HIBERNATION
+	depends on SUSPEND || HIBERNATION || XEN
 	default y
 
 config SUSPEND