Add VirDomain.GetCPUStats and VirDomain.GetInterfaceParameters

Added support for **virDomainGetCPUStats** and **virDomainGetInterfaceParameters**.
Dealing with the embedded union in the **virTypedParameter** struct makes for super ugly code. Also both those functions uses weird pointers everywhere, in the end I just did a 1:1 implementation, reflecting the quirks of the C version though I'd prefer if the Go version wouldn't have to deal with pointers.

I wrote two quick tests but disabled them as the vagrant wouldn't run on my linux (Connection refused on boot). Testing on my own KVM/libvirt setup yields correct results.

I'll see later to add the **virDomainGetMemoryParameters** function.