Distinguish between stopped pools and partially constructed pools in StoppedPools property

[mulkieran]

Note that this is a bit heavier weight than it looks, as if a pool went from partially set up to stopped, the StoppedPools property would have to be updated. We consider a pool to be partially set up but stopped if it can't be used, i.e., if it does not have a running thin pool.

If we did implement this, then stratis-cli would make use of it, and inform the users when they listed the stopped pools that some pools were not fully torn down.