docs: add Security chapter to the documentation

docs: add Security chapter to the documentation

This new chapter in the QEMU documentation covers the security
requirements that QEMU is designed to meet and principles for securely
deploying QEMU.

It is just a starting point that can be extended in the future with more
information.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Acked-by: Stefano Garzarella <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Li Qiang <[email protected]>
Message-id: 20190509121820[email protected]
Message-Id: <20190509121820[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>