We still point to git repositories hosted on libvirt.org in various
places. Replace the links to their gitlab.com equivalents.

Note that GitLab is trying to be smart here and
  https://gitlab.com/libvirt/libvirt
redirects to
  https://gitlab.com/libvirt/libvirt.git
when doing a 'git clone' and vice-versa when visiting from the
browser, so I only kept the .git suffix in places that explicitly
mentioned 'git clone'.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>