rpm-ostree: Correctly set fetching and transaction state

This is needed for proper update detection, especially in the updater:

- Make sure to keep the backend in the fetching state for the entire
  duration of the check for new updates.
- Make sure to update the state once a transaction is completed,
  canceled or fails.

Future changes will cache the "update found" state to avoid re-looking
for updates every time we setup the backend for each application
(discover, notifier, updater).