Something

-- qemu.sh --

#!/bin/sh
ip='/sbin/ip'
brctl='/usr/sbin/brctl'
cache='qemu-ifup.cache'
qemu='/usr/local/bin/qemu'

$ip addr flush dev eth0
$brctl addbr br0
$brctl stp br0 off
$brctl setfd br0 1
$brctl sethello br0 1
$ip addr add 192.168.0.2/24 broadcast 192.168.0.255 dev br0
$ip link set br0 promisc on
$ip link set br0 up
$brctl addif br0 eth0
$ip route add default via 192.168.0.1 dev br0
rm qemu-ifup.cache &>/dev/null
$qemu -hda hda.img -boot c -m 196 -localtime -n ./qemu-ifup.sh
$brctl delif br0 `head -1 qemu-ifup.cache` &>/dev/null
$brctl delif br0 eth0
$ip link set br0 down
$ip link set br0 promisc off
$ip addr flush dev br0
$brctl delbr br0
$ip route add default via 192.168.0.1 dev eth0

-- qemu-ifup.sh --

#!/bin/sh
ip='/sbin/ip'
brctl='/usr/sbin/brctl'
cache='qemu-ifup.cache'

$ip link set $1 up
$brctl addif br0 $1
echo "$1" > qemu-ifup.cache