s390x/cpu: Cleanup init in preparation for hotplug

Ensure a valid cpu_model is set upfront by setting the
default value directly into the MachineState when none is
specified.  This is needed to ensure hotplugged CPUs share
the same cpu_model.

Signed-off-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Message-Id: <1457112875-5209-2-git-send-email-mjrosato@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
