VMWare does not support --parallel

I am not sure if this is a bug or just undocumented, but neither VMWare Fusion or VMWare workstation support the `--parallel` flag. We have received a few support tickets around this /cc @mitchellh.
@sethvargo That is talking about something else. Running multiple in multiple terminals works. 

@uchagani The only official plugins are VirtualBox and VMware, there is no "list" right now.