Support for formatted overlayfs path spec when finding the root FS path

We're using snapshots in a CI set-up so that a Vagrant cluster can be built
once, then each push to the repository only checked as an incremental
update to the cluster. We copy each LXC VM to a master image, then re-create
the original names as snapshots.

This change corrects a method which assumes the LXC root path in the config
file is a simple directory name, which is only true for directory-backed
instances.