vl.c: add HMP help to machine

The help is based on the actual machine properties
exposing only the relevant options.

Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Greg Bellows <greg.bellows@linaro.org>
Message-id: 1418217570-15517-4-git-send-email-marcel.a@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
