Taken from upstream SVN, in scripts/qemu-ifup.

diff -ruN scripts-orig/qemu-ifup scripts/qemu-ifup
--- scripts-orig/qemu-ifup	1970-01-01 02:00:00.000000000 +0200
+++ scripts/qemu-ifup	2006-12-23 08:32:56.000000000 +0200
@@ -0,0 +1,5 @@
+#!/bin/sh
+
+switch=$(ip route list | awk '/^default / { print $NF }')
+ifconfig $1 0.0.0.0 up
+brctl addif ${switch} $1