Fixed shared gigabytes quota resource.

The shared gigabytes resource between volumes and snapshots wasn't
working properly.  The issue was that on update/sync the action item
(volumes or snapshots) would update the resource usages based only on
it's own particular item.

This patch fixes that, and makes the total gigabytes truly shared
between volumesa and snapshots.

Fixes bug: 1159489

Change-Id: Ib1be9788f0beb4f94d010e4f816f9f3393371205