Add DeviceSetWrapper

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)
Reuse a single DeviceSetDM for all the tests

We wrap the "real" DeviceSet for each test so that we get only
a single device-mapper pool and loopback mounts, but still
separate out the IDs in the tests. This makes the test run
much faster.