Add VirStream, VirDomainScreenshot, VirDomainSendKey

VirStream is libvirt's streaming implementation. Instead of recv and send methods, I interface with the io.Reader and io.Writer standard methods. I didn't implement everything, but this should cover most people's use cases.

VirDomainScreenshot takes a screenshot of the specified screen and sends it over to the client.
VirDomainSendKey sends keys to the virtual machine (keymappings are located [here](http://libvirt.org/git/?p=libvirt.git;a=blob_plain;f=src/util/keymaps.csv;hb=HEAD), and unfortunately only qemu is supported right now).