machine: remove qemu_machine_opts global list

QEMU has support for options per machine, keeping
a global list of options is no longer necessary.

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: 1418217570-15517-2-git-send-email-marcel.a@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>