Design sketch for quota support in reportstate command

This is stacked on top of #962, the design for the IFilesystem part of this feature.

This design proposes changes to make the volume manager spit out volumes that include size information inspected from the system and changes to make the reportstate implementation respect this information in the output it dumps.

This will eventually allow the convergence algorithm to deploy quota changes (by noticing the desired quota is different from the current quota).

`AttachedVolume` should only be a mount point and a reference to a `Volume`, yeah. So size will get moved into `Volume`.