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 wraps an existing DeviceSet and just adds a prefix to all ids in
it. This will be useful for reusing a single DeviceSet for all the tests
(but with separate ids)
This interface matches the device-mapper implementation (DeviceSetDM)
but is free from any dependencies. This allows core docker code
to refer to a DeviceSet without having an explicit dependency on
the devmapper package.
This is important, because the devmapper package has external
dependencies which are not wanted in the docker client app, as it
needs to run with minimal dependencies in the docker image.
