ISO upload, floppy and tools support, compatibility fixes

- The main commit is e308c0759f2f63516da9ada49e28eacf0533c052: new VDI code has been written, in the form of `step_{attach,detach,find,upload}_vdi`. As a consequence ISO and floppy upload is now working. Floppy attach should also work, when XAPI is patched to support them properly.
- Various config parameters have been renamed and tweaked to match what other builders take.
- Tweaks to ensure that Packer cleans up after itself more thoroughly.