VMware: Handle cases when there are no hosts in cluster

When there are no hosts defined in a cluster the host value will be an empty
string. For example:

    obj_context: (RetrieveResult){
       objects[] =
          (ObjectContent){
             obj =
                (obj){
                   value = "domain-c185"
                   _type = "ClusterComputeResource"
                }
             propSet[] =
                (DynamicProperty){
                   name = "host"
                   val = ""
                },
          },
     }

This causes an data access exception.

The code will now raise an NoValidHost exception.

Fixes bug: 1197041

Change-Id: I75b1f069e0b27eebd63c280e36533354c04e246a
Merge "VMware: Handle cases when there are no hosts in cluster"