Fixes the Hyper-V VolumeOpsTestCase base class

The VolumeOpsTestCase base class inherits incorrectly from
HyperVAPITestCase, with the unwanted side effect of executing all
base class tests as well.

This patch generalizes a common base class from HyperVAPITestCase
and fixes the inheritance issue.

Closes-Bug: #1280379

Change-Id: Ic81beb8397138c16c8d2d88a9c9d05d2b76e0e83
Merge "Fixes the Hyper-V VolumeOpsTestCase base class"