This moves Driver.Size() to Differ.DiffSize(), removing the empty
implementations in devmapper and dummy, and renaming the one in aufs.
Then we fall back to a container.Changes() implementation in the non-aufs
case.
We place the actual image/containers in the "rootfs" directory, which
allows us to have other data in the toplevel directory in the mount.
For starters, this means the "lost+found" directory from mkfs will
not always be in your container/image.
Secondly, we can create a file "id" in the toplevel dir which is not
visible from the container. This is useful because it allows us to map
back from the device fs to the container if something goes wrong with
the devicemapper metadata.
