croup: expose IO accounting data per unit

This was the last kind of accounting still not exposed on for each unit.
Let's fix that.

Note that this is a relatively simplistic approach: we don't expose
per-device stats, but sum them all up, much like cgtop does. This kind
of metric is probably the most interesting for most usecases, and covers
the "systemctl status" output best. If we want per-device stats one day
we can of course always add that eventually.