docs: Update godoc URLs for Go packages

docs: Update godoc URLs for Go packages

Since

  commit 201156ec6e65e08cdb04d6fdefe013652fb7fa87
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Mon Dec 16 10:06:50 2019 +0000

    docs: add pages to support Go module package resolution

it's possible to use

  import "libvirt.org/libvirt-go"
  import "libvirt.org/libvirt-go-xml"

in Go programs, which will result in the Go packages being
fetched directly from libvirt.org instead of GitHub.

The godoc website offers the API documentation for the updated
imports, and that's where we should send users.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>