Add a check for unlimited quotas

This extra check will prevent Horizon to
misunderstand "-1" value when tallying quotas,
setting the "Available" field to inf
instead of 0.

Change-Id: Idfeb9a10b8ce8eea1a39b3d48dffe08d6450fadd
Fixes: bug #1084976
Merge "Add a check for unlimited quotas" into stable/folsom