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

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

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20240109064628.595453-10-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

docs/devel/migration/features.rst

@@ -8,3 +8,4 @@ Migration has plenty of features to support different use cases.
 
    postcopy
    dirty-limit
+   vfio

docs/devel/migration/index.rst

@@ -10,6 +10,5 @@ QEMU live migration works.
    main
    features
    compatibility
-   vfio
    virtio
    best-practices