s390x/cpu: Add error handling to cpu creation

Check for and propogate errors during s390 cpu creation.

Signed-off-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
Message-Id: <1457112875-5209-7-git-send-email-mjrosato@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
