spapr: handle cpu core unplug via hotplug handler chain

Factor out cpu core unplug into separate function from
spapr_core_release(). Then use generic hotplug_handler_unplug() to trigger
cpu core unplug, which would call spapr_machine_device_unplug() ->
spapr_core_unplug() in the end.

This way unplug operation is not buried in spapr internals and located
in the same place like in other targets, following similar
logic/call chain across targets.

Acked-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>