Build-in websocket broken since v2.9.0-rc0

Since upgrading to 2.9.0, the qemu's build-in websocket was no longer available. 

Host: Ubuntu 16.04 LTS
Command-line: /bin/qemu-system-x86_64 -enable-kvm -vnc 0.0.0.0:8,websocket

I have tested the following qemu versions:

master      Fail
2.10.1      Fail
2.9.1       Fail
2.9.0       Fail
2.9.0-rc3   Fail
2.9.0-rc0   Fail

2.8.1.1     Pass
2.7.1       Pass
2.6.2       Pass