CPU hotplug is only possible on a per core basis on s390x. So let's
add possible_cpus and wire everything up properly.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170913132417.24384-19-david@redhat.com>
Acked-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>