CONFIG_PM_SLEEP fix: xen: fix compilation when CONFIG_PM_SLEEP is disabled

Xen save/restore depends on CONFIG_PM_SLEEP being set for device_power_up/down.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>