s390x: CPU hot unplug via device_del cannot work for now

s390x: CPU hot unplug via device_del cannot work for now

device_del on a CPU will currently do nothing. Let's emit an error
telling that this is will currently not work (there is no architecture
support on s390x). Error message copied from ppc.

(qemu) device_del cpu1
device_del cpu1
CPU hot unplug not supported on this machine

Reviewed-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170913132417.24384-18-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>