stratisd does not detect manually torn down devicemapper devices

[jbaublitz]

This problem is more cosmetic, but we may want to handle it in the future.

If a pool is left in a partially torn down state, and then the user tears down the remaining devices, stratisd does not detect that the devices have been manually torn down and leaves the pool in the "partially constructed pool" slot.

On any udev event affecting the pool or if the user issues a stratis pool stop command on the partially constructed pool, this should move the pool to the correct "stopped pool" slot.