Fixes an get_volume_stats reporting issue

This patch fixes a bug introduced in the last get_volume_stats
patch that caused the method to fail.  I Also added another set
of unit tests to catch any future issues related to get_volume_stats.

Fixes Bug: #1178418

Change-Id: I0dd2e0113ff822d3e717ab3d6da6b9d1b914a995
Merge "Fixes an get_volume_stats reporting issue"