Track block objects explicitly.

So that our picture always matches UDisks.  Otherwise we might forget
about blocks when they are unpublished, and will never publish them
again.

* Keep a hashtable in UlManager that tracks UDisks block objects.
* Turn UlBlock into a plain GObject and adjust all users.
* Update a newly created UlBlock from all existing volume groups to avoid
  a race with UDisks.

Signed-off-by: Stef Walter <stefw@redhat.com>