Point to the new libvirt-go bindings

Point to the new libvirt-go bindings

The github.com/rgbkrk/libvirt-go bindings were the most complete
bindings historically, but their API coverage stops at 1.2.4,
with exception of a couple of newer APIs.

The new bindings at http://libvirt.org/git/?p=libvirt-go.git;a=log
how have (almost[1]) 100% API coverage all the way to 2.5.0. They also
expose the APIs in a way that allows for much stronger go type
checking by the compiler, and expose typed parameters as explicit
structs. Finally the bindings are able to conditionally compile against
any libvirt version 1.2.0 -> 2.5.0 without use of go build tags.

Change the docs to point to these new bindings, since they'll be
a better bet for users long term.

[1] virEvent & virStream callbacks are still TODO to be fixed
    real soon.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>