ppc: Rename 2.13 machines to 3.0

Rename the 2.13 machines to match the number we're going to
use for the next release.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-id: 20180522104000.9044-5-peter.maydell@linaro.org