virtio-ccw: enable virtio-1

Let's enable revision 1 for virtio-ccw devices. We can always offer
VERSION_1 as drivers in legacy mode won't be able to see it anyway.

We have to introduce a way to set a lower maximum revision for a device
to accommodate the following cases:
- compat machines (to enforce legacy only)
- virtio-blk with scsi support (version 1 + scsi is fenced by common
  code, with a user-configured max revision of 0 we can allow scsi
  via not offering VERSION_1)

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>