[Libguestfs] virtio is enabled by default in libguestfs?

Hi all, Could I ask if and from which version that virtio is enabled by default in libguestfs? It seems that it is enabled by default in fedora-12, on which the latest version of libguestfs is 1.0.85. But for fedora-11(the latest version of libguestfs is 1.0.72), I could enable virtio only when I set it explicitly via config command. Thus it is not enabled by default on fedora-11. So I'd like to know on which situation virtio will be enabled automatically by libguestfs? Thanks. --Yufang