docs: add Secure Coding Practices to developer docs

At KVM Forum 2018 I gave a presentation on security in QEMU:
https://www.youtube.com/watch?v=YAdRf_hwxU8 (video)
https://vmsplice.net/~stefan/stefanha-kvm-forum-2018.pdf

 (slides)

This patch adds a guide to secure coding practices.  This document
covers things that developers should know about security in QEMU.  It is
just a starting point that we can expand on later.  I hope it will be
useful as a resource for new contributors and will save code reviewers
from explaining the same concepts many times.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-id: 20190509121820.16294-2-stefanha@redhat.com
Message-Id: <20190509121820.16294-2-stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>