To get VNC going, make sure you have a VNC graphics device with websockets enabled, like so:
<graphics type='vnc' port='-1' autoport='yes' websocket='-1' listen='0.0.0.0' passwd='somethingcomplexrandom'>
<listen type='address' address='0.0.0.0'/>
</graphics>
The reason -1 is used for ports is to let libvirt figure out a port and dynamically assign one. If you prefer, you can statically assign unique ports of your choosing. You will not need to know the ports or the password (use something complex/random) when using WebVirtMgr. The password is important (not that it represents real security) because there's no actual proxy being used here, so VNC ports will be exposed to the entire network.
To be clear, you do not need websockify at all when using the aforementioned approach. Websockify exists to convert a TCP socket into a web socket. QEMU has supported directly exposing VNC as a websocket for some time now, which is all we're enabling here.