VMware Driver update correct disk usage stat

The nodes managed by VMware VCDriver driver can have multiple
datastores (disk abstraction) attached to it. The driver currently
reports the disk usage of a datastore that has the largest capacity.
But it doesn't report a consolidated disk usage. As a result, the
node's storage capability is under-reported in many occassions.

This patch fixes this issue by aggregating the disk freespace,
capacity from all attached datastores.

Closes-Bug: #1220459

Change-Id: I818a041f33a045773fc799de2c69628064ef8cbf
Merge "VMware Driver update correct disk usage stat"