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
(cherry picked from commit d6935af9b3cdf73cc5dac2006d76a95e4c6cb35e)