s390x/s390-stattrib: Mark the storage attribute as not user_creatable

The storage attribute devices are only meant to be instantiated one
time, internally. They can not be used by the user, so mark them with
user_creatable = false.

Suggested-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1503576029-24264-1-git-send-email-thuth@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>