`export DISPLAY=:0` did the trick.
The following shows when still on poky dunfell:
* I can now boot `runqmu kvm slirp sdl core-image-weston`. But gui is still slow. I cannot even type `systemctl poweroff` in weston-terminal, it inevitably becomes `wweesssttooo[...]`.
* `runqemu kvm slirp sdl gl core-image-weston` still fails with `Failed to run qemu: qemu-system-x86_64: OpenGL support is disabled`.
Updated to current poky master, but got this: `ERROR: ParseError in configuration INHERITs: Could not inherit file classes/image-mklibs.bbclass`.
Switched to hardknott-3.3.3 which is four weeks old and perhaps a little riper. It's currently building.