s390x/migration: Introduce 2.4 machine

The section footer changes commit f68945d42bab ("Add a protective
section footer") and commit 37fb569c0198 ("Disable section footers
on older machine types") broke migration for any non-versioned
machines.

This pinpoints a problem of s390-ccw machines: it needs to
be versioned to be compatible with future changes in common
code data structures such as section footers.

Let's introduce a version scheme for s390-ccw-virtio machines.
We will use the old s390-ccw-virtio name as alias to the latest
version as all existing libvirt XML for the ccw type were expanded
by libvirt to that name.

The only downside of this patch is, that the old alias s390-ccw
will no longer be available as machines can have only one alias,
but it should not really matter.

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Cc: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
Cc: Jason J. Herne <jjherne@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <1435742217-62246-1-git-send-email-borntraeger@de.ibm.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
