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.
Upstream-commit: 5d76681c3d0cbb744205a397420603ce029ce754
Component: engine
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.
Upstream-commit: 00401a30b73d897255b7b99996d7c6a53fe2d4af
Component: engine
