Description of problem:

In cloudforms dashboard. A virtual machine's disk information tab has "Provisioned Size", "Used Size" and "Percent Used of Provisioned Size" 
fields among others. The Provisioned Size of virtual disk is shown correctly but "Used Size" is not captured and its always blank due to which "Percent Used of Provisioned Size" is also not correctly populated. 

Collecting "used size" information is very important for storage capacity planning. thin provisioned disk actual disk usage should shown as "Used Size"
and there should be a filter given to create a report based on "Used Size"
of disks. 

Version-Release number of selected component (if applicable):
5.5.0.13.20151201120956_653c0d4
RHEV 3.5 and later

How reproducible:
always 

Steps to Reproduce:
1. Go to virtual machine tab and select a virtual machine to open its property page. Look for "Datastore Allocation Summary" section and click the disk icon
in it.
 
2. "Number of Disks" for Virtual Machine "vm" page will open. Notice "Used Size" is set to blank and Percent Used Provisioned Space: is always 0.0 %

Actual results:

No used size information captured 

Expected results:

A disk's "used size" information should be captured

Additional info:

qemu-img info 5360be1d-4a2c-4623-be3e-3c4e81bf22a3
image: 5360be1d-4a2c-4623-be3e-3c4e81bf22a3
file format: raw 
virtual size: 10G (10737418240 bytes)      < should shown as provisioned disk
disk size: 1.8G < should shown as used size