devmapper: Use a "rootfs" subdirectory in the devmapper volume

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.
Merge pull request #43 from alexlarsson/dm-plugin-use-root-subdir

devmapper: Use a "root" subdirectory in the devmapper volume