I feel the urge to point out that QEMU itself does not do "the networking". That's different from "all in one solutions" like VirtualBox. Instead, you set up your networking using common tools like ip(8) and the already mentioned brctl(8). You then only tell QEMU to use what you've set up.

At least, that's how I do it in overqemu: https://github.com/vain/bin-pub/blob/master/overqemu

QEMU also provides the option of building "multicast networks" where you don't have to do any setup in advance. That's easy to use but reeeeeally slow.