docs: point users to gitlab for issue tracking

docs: point users to gitlab for issue tracking

Currently we use the "Virtualization Tools" product in Red Hat Bugzilla
for issue tracking upstream. This changes to point people to GitLab for
issue tracking.

Note that Bugzilla still has plenty of bugs present against libvirt.
Triaging these to determine what is still valid will be a separate
exercise. Bugzilla will be locked to prevent creation of new issues
meanwhile.

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