Thanks all for your support. Finally it works. The config was described many times in several posts here in forum:
<serial type="dev">
<source path="/dev/serial/by-id/usb-dresden_elektronik_ingenieurtechnik_GmbH_ConBee_II_DEXXXXXXX-if00"/>
<target type="usb-serial" port="1">
<model name="usb-serial"/>
</target>
<address type="usb" bus="0" port="4"/>
</serial>
BUT
the very important part was not to only restart KVM instance, but power off and then start it. I was only restarting it before and the usb-serial was not visible even it was present in KVM XML config. It must be properly shut down and then started and after that it was visible in HAOS. Maybe some glitch in KVM.
Stick is then visible as
/dev/serial/by-id/usb-QEMU_QEMU_USB_SERIAL_1-0000:00:03.7-4-if00-port0
in HA as mentioned earlier here.