sudo snap connect cups:avahi-control
and rebooting the virtual machine the auto discovery of the shared printers works again, in both directions. So after manually connecting Avahi the CUPS/Avahi/D-Bus/cups-browsed realm seems to fully work with a confined snap.
Can I reach the same goal also without rebooting the VM? How do I restart daemons within a snap?
We also need a way to connect the Avahi interface automatically.