docs: point to GitLab as the primary git hosting

docs: point to GitLab as the primary git hosting

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: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>