vmware: driver races to create instance images

Addresses race condition that occurs when multiple processes
arrive at a fresh Datastore with no vmware_base cache directory.

Change-Id: If1bb6804e3668a56872a653a8ea44d0c7fc70311
Co-Authored-by: Tracy Jones <tjones@vmware.com>
closes-bug: #1284312
Merge "vmware: driver races to create instance images"