virtio-ccw: change realization sequence

virtio-ccw has an odd sequence of realizing devices: first the
device-specific relization (net, block, ...), then the generic
realization. It feels less odd to have the generic realization
callback trigger the device-specific realization instead (and this
also matches what virtio-pci does).

One thing to note: We need to defer initializing the cu model in the
sense id data until after the device-specific realization has been
performed, as we need to refer to the virtio device's device_id.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Message-Id: <1429627016-30656-2-git-send-email-cornelia.huck@de.ibm.com>