Right now this does nothing but add a new layer, but it means
that all DeviceMounts are paired with DeviceUnmounts so that we
can track (and cleanup) active mounts.
This is a module that uses the device-mapper create CoW snapshots
You instantiate a DeviceSetDM object on a specified root (/var/lib/docker),
and it will create a subdirectory there called "loopback". It will
contain two sparse files which are loopback mounted into
a thin-pool device-mapper device called "docker-pool".
We then create a base snapshot in the pool with an empty filesystem
which can be used as a base for docker snapshots. It also keeps track
of the mapping between docker image ids and the snapshots in the pool.
Typical use of is something like (without error checking):
devices = NewDeviceSetDM("/var/lib/docker")
devices.AddDevice(imageId, "") // "" is the base image id
devices.MountDevice(imageId, "/mnt/image")
... extract base image to /mnt/image
devices.AddDevice(containerId, imageId)
devices.MountDevice(containerId, "/mnt/container")
... start container at /mnt/container
