Installing QEMU VMs through Cockpit is not straightforward.
You still need additional configuration.
Here is the rough draft of what I did to get it working.
- setup bridge interface using standard linux setup.
- install brctl, dnsmasq, and ebtables.
- run virsh edit to modify the VNC port.
sources:
https://forum.openmediavault.org/index.php/Thread/26913-Cockpit-VirtMachine-Error/
https://forum.openmediavault.org/index.php/Thread/25143-Cockpit-Qemu-and-VirtMachine/
https://qemu.weilnetz.de/doc/qemu-doc.html