VMware: Check for the propSet attribute's existence before using

There may be cases when the call to RetrievePropertiesEx may have missing
data. That is, the propSet field may be missing. In this case the missing
field will be logged. This will help with troubleshooting the problem.

Fields may be missing due to one of the following reasons:
 - SystemError if there was some unknown problem reading the value
 - NoPermission if the logged in session did not have permission to read
   the value

The propSet attribute is marked as optional in the API documentation so this
should have been anticipated and this can be interpreted as a bug in the
client code.

http://pubs.vmware.com/vsphere-51/index.jsp#com.vmware.wssdk.apiref.doc/vmodl.query.PropertyCollector.ObjectContent.html

Fixes bug: 1215958

Change-Id: If8fd912327e057320cd6013c276000617b9c320c
Merge "VMware: Check for the propSet attribute's existence before using"