Added libvirt event support using libuv

Hello,

this is the implementation and registration of libvirt event handler. I Didn't test it a lot .. but it works for the simple cases i've tested.
Here is a very small code example, note the setupEvent() call, it registers event callbacks into libvirt :

    var libvirt = require('libvirt');
    libvirt.setupEvent();
    
    var hv = new libvirt.Hypervisor('qemu:///system');
    
    hv.registerDomainEvent({
      evtype: 0,
      callback: function(hv, domain, data) {
        console.log("state change " + domain.getName() + " " + data.evtype + " " + data.detail);
      }
    });

this is the console output for a VM i've started then forced to shutdown :

    state change debdev 2 0
    state change debdev 5 1

Anthony.