This should work as far as Plan 9 is concerned. It's qemu that is troublesome. One possible approach to debugging is to boot the live (install) CD and then run ip/ipconfig -g 10.0.2.2 ether /net/ether0 10.0.2.15 255.255.255.0 and see if that's enough to get a working network (i.e., can you ping or telnet to port 567 or 9fs the remote machine).
I have this script as my /etc/qemu-ifup, though I can't remember exactly why it is needed: #!/bin/sh sudo -p "Password for $0:" /sbin/ifconfig $1 172.20.0.1 Russ
