Added Dockerfile to run the tests.

**Disclaimer:** *I am aware that this commit may be considered useless by vagrant fans, however, vagrant+virtualbox is a nogo on my dev machine running KVM.*

Vagrant has the inconvenient to be depending on VirtualBox, while
docker is just easy to setup. To run the tests, just run those two
commands:

    sudo docker build -t libvirt-go:master .
    sudo docker run -ti --rm libvirt-go:master

This could be made even easier through a simple Makefile