VMware: Only include connected hosts in cluster stats

Currently the driver includes all hosts connected to VC
(including disconnected hosts) when calculating available
vcpus. The patch enforces that the reported available stats
only include resources of hosts that are connected to the
VC.

Note: A host can be connected to a VC, but have a disconnected
host state

Change-Id: Iba9d80f894cc2b7471a7f9c87b7367fd93bacb48
Closes-Bug: 1257998
Merge "VMware: Only include connected hosts in cluster stats"