Fix overflow problem when connecting to multiple PowerVC hypervisors

The value of the property 'free_disk_gb' in the hypervisor info is
invalid in the Powervc. And the PowerVC will set it as a big value.
It will lead the overflow of the database.

Change-Id: I0eae9f393cd1c191230c71687450318539d58e02
Closes-Bug: 1370895