Description of problem:
When the LXCA doesn't provide Disk Capacity information, the ManageIQ is saving with 0, instead of leaving nil.


How reproducible:
Manage an LXCA with a version lower than 2.0

Steps to Reproduce:
1. Access the physical server detail page;
2. Notice the Disk Capacity field.

Actual results:
The value is 0

Expected results:
Must be empty