VMware: Remove ds_util.build_datastore_path()

Convert all remaining users of build_datastore_path to use the
DatastorePath class, and remove build_datastore_path() and its tests.

Change-Id: I73a166a48c0038743213e4feaa95834f9ebc8fbf
Merge "VMware: Remove ds_util.build_datastore_path()"