virtio: move common virtio properties to bus class device

The two common virtio features can be defined per bus, so move all
into bus class device to make code more clean.

As discussed with cornelia, s390-virtio-blk doesn't support
the two features at all, so keep s390-virtio as it.

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com> #for s390 ccw
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

MST: rebase and resolve conflicts