Address potential races in SolidFire VAG

Address potential races in SolidFire VAG

While building the initial volume access group solution I realized there were
race conditions present. This patch attempts to address those by being far
more careful during VAG creation and removal. Tracking multiattach volume
detaches around VAG volume removal introduced all sorts of future race
conditions so instead multiattach volumes are treated differently and are only
removed from VAGs on volume removal. Not ideal but it'll get the job done.

Think I covered all the new branches with unit tests. Complexity was introduced
but it should be worth it due to the safety that was also introduced.

Change-Id: I80034cdf759295c8d379c02ed9e42659590b41a9