Break out over-quota calculation code from quota_reserve()

The list comprehension used to create the overs list was really hard to
read, so this moves that code into it's own helper method and breaks out
the logic from the list comprehension, and adds unit tests.

Partial-Bug: #1369605

Change-Id: I44236c7a206b2a9bd3843c9ca2ce56591a66352f
Merge "Break out over-quota calculation code from quota_reserve()"