Added Dockerfile to run the tests.

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 --privileged libvirt-go:master go test

This could be made even easier through a simple Makefile
Integration test: fixed GetCPUStats to depend on the host number of CPUs

This fix allows the test to work even on machines that have more than
one CPU. This is needed for the integration tests to pass when running
in a docker container.