Add Instructions and Fixes for Virtualbox-Only Use

[jmchilton]

From IRC:

17:02 < __jxtx__> Anybody had success with planemo-machine virtualbox-iso on mac os x? SSH always fails to 
                  authenticate. Increased the timeout, made headless false so I could see the console, but 
                  it shuts down when it fails so I can't see any logs really.
17:05 < dannon> Hrmm, I've been running it on my linux box, let me try the mac.
17:05 < jmchilton> __jxtx__: are you using it with or without vagrant?
17:07 < __jxtx__> without, just: packer build --only="virtualbox-iso" packer.json
17:07 < jmchilton> oh - you are trying to build it?
17:08 < __jxtx__> Yes. I could probably vagrant and extract from the box, but my end goal is an OVA image 
                  that can run without Vagrant, just VirtualBox alone
17:08 < jmchilton> sure - that would be cool
17:08 < __jxtx__> Is this an untested use?
17:09 < jmchilton> I can only state I have only ever used it to build vagrant boxes.
17:09 < dannon> I also used vagrant, and of course google compute images.
17:10 < dannon> Trying virtualbox now -- if it doesn't work it should, and we can fix it.
17:11 < __jxtx__> could be something as simple as username? that seems like the only difference in the 
                  vagrant...sh script -- packer build -only virtualbox-iso -var 'username=vagrant' 
                  $PACKER_CONF
17:12 < dannon> Mine's taking forever to build.  I didn't run headless, but I can see it actually still 
                doing stuff in the little virtualbox preview window
17:12 < dannon> Err, didn't run with headless=false.
17:12 < __jxtx__> mine seemed to finish the ubuntu install (or it was, then I looked away).

[dannon]

Initial tweaks in vbox_only_fixes branch.

[jmchilton]

make virtualbox. README updated.