Having dissected the debug logs and attempted to run the qemu command manually I get an error about Device user could not be initialized. Is there something I'm missing?
root@packer:~# /usr/bin/qemu-system-x86_64 -no-reboot -drive file=/root/test/tdhtest.qcow2,if=virtio -display none -device virtio-net,netdev=user.0 -netdev user,id=user.0 -redir tcp:2228::22 -name tdhtest -machine type=pc-1.0,accel=kvm -cdrom /home/packer/CentOS-6.4-x86_64-bin-DVD1.iso -boot d -m 512m
qemu-system-x86_64: -netdev user,id=user.0: could not set up host forwarding rule 'tcp:2228::22'
qemu-system-x86_64: -netdev user,id=user.0: Device 'user' could not be initialized