Support for Ubuntu 12.10 desktop

This is a template for Ubuntu 12.10 desktop for amd64, and a couple of changes to make veewee start VMs with 3d acceleration, which is necessary for Ubuntu 12.10 desktop to work.

My first approach was to try the Ubuntu-desktop ISO, but it seems to be very different from the server ISO, and I couldn't even get it to start the installation, even after a lot of playing with the boot options. The best I could do was get it to boot into a bare session with busybox.

Notes:

* This installs a base install off the server ISO, and then adds the ubuntu-desktop task, using `tasksel`. I tried adding `ubuntu-desktop` to the `preseed.cfg` file, but it didn't seem to have any effect (perhaps because those files aren't on the ISO image, and the network isn't availabe at that stage), so instead I added `tasksel install ubuntu-desktop` to the `postinstall.sh`.
* I had to enable 3D acceleration to make X start. It isn't just Unity that wouldn't start -- running `X` resulted in an error of "no displays found". This option is only set for Virtualbox.
* If this were to be made to work on other virtualization platforms,  `'on'` and `'off'` are probably not good values to use for `video_acceleration_3d`. Right now these values are passed directly to the `--accelerate3d` option with `VBoxManage`, but "on" and "off" likely aren't the right options for other virtualization platforms.
* The desktop template probably won't work for VMWare and KVM, since X apparently won't start without 3D acceleration.
* Is it possible to change the default settings in the Vagrantfile created by `vagrant init ubuntu-12.10-desktop-amd64`? It would be nice if this had `config.vm.boot_mode = :gui` by default, since this is a desktop system.


If this approach looks good, I can duplicate it for i386, and try it for Ubuntu 12.04 as well. I'm definitely open to suggestions of how to improve it or make it fit better with existing patterns.