Adjust audit logs to avoid negative disk info

When requesting more disk than is available, the audit
logs will report negative values. This may confuse the
user or make him/her think the tracking is innaccurate.

This patch seeks to remove any confusion. It is in the
same vein as https://review.openstack.org/#/c/93261/
which addresses the same issue in regards to cpu and
memory audit logs.

Change-Id: I184985573b20144717ac40f1f3c26709030d5a03
Closes-bug: #1350542
Merge "Adjust audit logs to avoid negative disk info"