x86, xen, power: fix up config dependencies on PM

Xen save/restore needs bits of code enabled by PM_SLEEP, and PM_SLEEP
depends on PM.  So make XEN_SAVE_RESTORE depend on PM and PM_SLEEP
depend on XEN_SAVE_RESTORE.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>