s390x: fix cpu object referrence leak in s390x_new_cpu()

object_new() returns cpu with refcnt == 1 and after realize
refcnt == 2*. s390x_new_cpu() as an owner of the first refcnt
should have released it on exit in both cases (on error and
success) to avoid it leaking. Do so for both cases.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1508247680-98800-2-git-send-email-imammedo@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
