All the command before qemu booted.
qemu-img create hd_img.img 8000M

modprobe tun

chmod 666 /dev/net/tun

modprobe kqemu

chmod 666 /dev/kqemu

tunctl -d tap0

tunctl -u joe -t tap0
/etc/qemu-ifup tap0

#!/bin/sh

sudo -p "Password for $0:" /sbin/ifconfig $1 172.20.0.1
echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -t nat -A POSTROUTING -o eth0 -s 172.20.0.0/24 -j MASQUERADE
The boot command:
qemu -m 300 -net nic,vlan=0 -net tap,vlan=0,ifname=tap0 -hda

hd_img.img -cdrom install43.iso -boot d
install43.iso is the 4.3openbsd.

When coming to the following line at the boot time,the guest system stoped.

root on rd0a swap on rd0b dump on rd0b

That is the last dmesg line.