Ubuntu customization support in Oz

Hey Steve, So I finally found a bit of time to go through your patches. They looked very similar to the ones I had initially come up with, but you ended up finding a good way around the two issues that I initially had[1]. I did a bit of cleanup on the patches. First, I made the comments of the setup/teardown consistent with what the code was actually doing. Second, I fixed up the imports on Ubuntu.py (your patch had an unnecessary import guestfs, and was missing an import libvirt). Finally, I merged the code that I had for doing icicle generation with your code, so in theory that should now work. I've now pushed your patches. I have not had time to test them yet, so if you get a chance please do so and let me know. Thanks for getting this done, it was in the "half-completed" state for a long time and it is good to finally commit it. Thanks, Chris Lalancette