VMware: ensure that resource exists prior to accessing

Ensure that the resource being accessed actually exists. There
may be cases when a cluster is updated/deleted and the scheduler
may be out of sync with the compute node.

Fixes bug: 1216510

Change-Id: Ib82af10f78fbad0cec55eff3dffd3d36409d8b24
Merge "VMware: ensure that resource exists prior to accessing"