s390x: add css_migration_enabled to machine class

s390x: add css_migration_enabled to machine class

Currently the migration of the channel subsystem (css) is only partial
and is done by the virtio ccw proxies -- the only migratable css devices
existing at the moment.

With the current work on emulated and passthrough devices we need to
decouple the migration of the channel subsystem state from virtio ccw,
and have a separate section for it. A new section  however necessarily
breaks the migration compatibility.

So let us introduce a switch at the machine class, and put it in 'off'
state for now. We will turn the switch 'on' for future machines once all
preparations are met. For compatibility  machines the switch will stay
'off'.

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Message-Id: <20170711145441.33925-3-pasic@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>