Expose more information about blockdev usage on the D-Bus

[mulkieran]

Right now, we only expose one key, which represents total size. We would like to expose more information about blockdev usage via the FetchProperties interface.

[mulkieran]

And we would like to be able to report it in the CLI.

[mulkieran]

In the CLI, it seems reasonable to report Total/Used/Free as we do in the pool listing.

[mulkieran]

Blocked by #300

[mulkieran]

Right now, stratisd can only recognize the new device size if it is brought down and then back up again. It also wouldn't be expected to work on pools with encrypted devices. So, we can only test a few simple things:

1. Test that stratisd recognizes the new larger size.
   i. Create a new pool, unencrypted, with a least one device, a dm-raid device.
   ii. Verify that the real size and the size on the metadata are the same. In this case, the CLI will show just the one size.
   iii. Change the size of the dm-raid device so that it is larger by some amount.
   iv. Stop stratisd and restart it.
   v. Verify that the new size of the devices has been detected. The CLI should show both sizes when listing the devices.

2. Test that stratisd refuses to create a pool if the device is smaller than the metadata specifies.
   i. Create a new pool, unencrypted, with a least one device, a dm-raid device.
   ii. Verify that the real size and the size on the metadata are the same. In this case, the CLI will show just the one size.
   iii. Change the size of the dm-raid device so that it is smaller by some amount.
   iv. Stop stratisd and restart it.
   v. Verify, using the report interface, that stratisd has put the set of devices into its errored pools field.

In both tests, inspect the stratisd logs. The liminal module reports, at the debug level, every occasion on which it picks up a relevant change event, and it probably will pick up a change event when the device's size changes. I think it also makes sense to run udevadm monitor when changing the device size to identify precisely the events that are detected. I believe that in the general case, though, we will not be able to rely on a udev change event to detect a device size change.