Document how to use QEMU user mode networking with passt

QEMU's user-mode networking currently defaults to slirp. In recent times, there's an alternative to slirp called passt and it officially supports QEMU. They even have a little demo to show this off. I think it would be neat to document this possibility in the QEMU docs themselves? Currently, this alternative is a little hard to discover if you don't know what you're looking for.