VMware: refactor get_datastore_ref_and_name

Update vm_util.get_datastore_ref_and_name to use Datastore class,
renamed it to get_datastore, remove vm_util.DSRecord along the way.

partial blueprint vmware-spawn-refactor

Change-Id: Ib3308870c09344036c4f330a48513581f5771246
Merge "VMware: refactor get_datastore_ref_and_name"