Move it one layer down, so taking Virtio-migration as a feature for
migration.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20240109064628.595453-11-peterx@redhat.com


Signed-off-by: Peter Xu <peterx@redhat.com>