Description of problem:
When using CFME in combination with RHEV, the "Used" storage per VM is not shown correctly. 

Although CFME detects that the VM is using Thin Provisioning, the "Used" storage always matches the "Allocated" storage.

Version-Release number of selected component (if applicable):
5.5.2

How reproducible:
tested with RHEV 3.4 and RHEV 3.5

Steps to Reproduce:
1. add a RHEV provider
2. make sure there is a least one VM with "thin provisioning"
3. navigate to the VM details and check the "Datastore Allocation Summary" and "Datastore Actual Usage Summary" section

Actual results:
"Used" always matches "Allocated"

Expected results:
"Used" should match the Actual usage shown in the RHEV UI.

Additional info:
Only tested with NFS storage!