VMware: Fix deletion of an instance with no files

When a managed object doesn't contain a property, RetrievePropertiesEx
doesn't return None; it doesn't return the property at all. We were
testing for this incorrectly and not handling it in the code.

Change-Id: I77a0ba4eb5a6cc6075db9c67294f30abaa0e474d
Merge "VMware: Fix deletion of an instance with no files"