I agree with avoiding floppy. So, perhaps a new source uri like: 9p://tag/path, where "tag" is the virtio device tag and "path" is the path within it.
Regarding the modules:
$ find /lib/modules/$(uname -r)/ | grep 9p
/lib/modules/3.16.0-4-amd64/kernel/net/9p
/lib/modules/3.16.0-4-amd64/kernel/net/9p/9pnet_rdma.ko
/lib/modules/3.16.0-4-amd64/kernel/net/9p/9pnet.ko
/lib/modules/3.16.0-4-amd64/kernel/net/9p/9pnet_virtio.ko
/lib/modules/3.16.0-4-amd64/kernel/fs/9p
/lib/modules/3.16.0-4-amd64/kernel/fs/9p/9p.ko
I can't see why 9pnet_rdma would be needed, so it's 9p.ko, 9pnet.ko and 9ipnet_virtio.ko. 9p also needs virtio_ring and virtio but these are already available.