target/s390x: change CPU type name to "s390x-cpu"

For now, e.g. host-s390-cpu wasn't exposed to the user. cpu-add, -cpu
and the CPU model qmp interfaces didn't care about the actual type,
as that information was hidden.

This changed with CPU hotplug via device_add. Now the type is visible to
the user. Before we get that supported in a stable version, this is our
last chance to change it.

So change it from "s390-cpu" to "s390x-cpu", to match the architecture
name. Example names are then e.g. z14-s390x-cpu or qemu-s390x-cpu.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171020115803.14093-1-david@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
