Domain events API implementation

The big amount of callbacks from libvirt C API has been mapped to
different details structs in the Go API.
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 libvirt-go:master

This could be made even easier through a simple Makefile