I believe vlan is being deprecated:
https://www.linux-kvm.org/page/Networking

With qemu 2.11.1, I get the following warning:
15:25:50: INFO: RunCommand: /usr/local/google/home/achuith/code/qemu/build/qemu-2.11.0-rc3/build/x86_64-softmmu/qemu-system-x86_64 -m 2G -smp 4 -vga virtio -daemonize -pidfile /tmp/cros_vm/kvm.pid -chardev 'pipe,id=control_pipe,path=/tmp/cros_vm/kvm.monitor' -serial file:/tmp/cros_vm/kvm.monitor.serial -mon 'chardev=control_pipe' -net 'nic,model=virtio,vlan=9222' -net 'user,hostfwd=tcp:127.0.0.1:9222-:22,vlan=9222' -drive 'file=/usr/local/google/home/achuith/code/chrome/.cros_cache/chrome-sdk/tarballs/amd64-generic+10171.0.0+chromiumos_qemu_image.tar.xz/chromiumos_qemu_image.bin,index=0,media=disk,cache=unsafe,format=raw' -enable-kvm
qemu-system-x86_64: -net nic,model=virtio,vlan=9222: 'vlan' is deprecated. Please use 'netdev' instead.