cpupower: Better detect offlined CPUs

Before, checking for offlined CPUs was done dirty and
it was checked whether topology parsing returned -1 values.
But this is a valid case on a Xen (and possibly other) kernels.

Do proper online/offline checking, also take CONFIG_HOTPLUG_CPU
option into account (no /sys/devices/../cpuX/online file).

Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>