virtio_balloon: correct tags for config space fields

Tag config space fields as having little endian-ness.
Note that balloon is special: LE even when using
the legacy interface.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>