VMware vSphere support: Disk rates

1) Implemented pollsters for following disk rate counters
   - disk.read.bytes.rate
   - disk.read.requests.rate
   - disk.write.bytes.rate
   - disk.write.requests.rate

2) Extended the 'virt.inspector' to return the disk rate stats via
'inspect_disk_rates()'. As of now only vmware.inspector implements this
method.

3) Wrote test cases for the newly added disk rate pollsters and
inspect_disk_rates().

Change-Id: I1089b708ae3b651c95013129dbc0129b419d2b62
Implements: blueprint vmware-vcenter-server
Merge "VMware vSphere support: Disk rates"