docs/migration: Further move virtio to be feature of migration

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>

docs/devel/migration/features.rst

@@ -9,3 +9,4 @@ Migration has plenty of features to support different use cases.
    postcopy
    dirty-limit
    vfio
+   virtio

docs/devel/migration/index.rst

@@ -10,5 +10,4 @@ QEMU live migration works.
    main
    features
    compatibility
-   virtio
    best-practices