VMware: use datastore classes in file_move/delete/exists, mkdir

Straightforward conversion of a few functions in ds_util to
accepts DatastorePath objects instead of literal strings.

partial blueprint vmware-spawn-refactor
Change-Id: I9dff390ce020dcc39a05184a7ee69902e3412c9f
Merge "VMware: use datastore classes in file_move/delete/exists, mkdir"