hw/i386: Rename 2.13 machine types to 3.0

Rename the 2.13 machine types to match what we're going to
use as our next release number.

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: Eduardo Habkost <ehabkost@redhat.com>
Message-id: 20180522104000.9044-3-peter.maydell@linaro.org