Compact disk (with some caveats)

Per #84, this implements VMWare disk compaction as the last build step.

Don't merge this quite yet, though - in my testing, if you are already zeroing out the disk, this will actually make the final image a little larger, like so: 

```
$ du -h packer_vmware.box packer__vmware.box
591M    packer_vmware.box (built from binaries compiled from ac5c90bd7ea2d292777530edde66ce450247d90c)
600M    packer__vmware.box (built from my feature branch)
```

I'm testing the difference now if you don't bother zeroing out the disk. Hopefully this PR was not in vain, or someone else can weigh in on what we might want to do differently.