How to boot a UEFI image from a KVM virtual machine without `virt-manager`? (e.g. Home Assistant on OpenMediaVault/Cockpit)

Disclaimer: I am not really familiar with UEFI, don't even know exactly what it does, so if this answer can be improved, please comment or edit accordingly.

This is what worked for me:

apt-get install ovmf

cd /usr/share/ovmf/

Check that the file OVMF.fd is there, because that is what libvirt needs to boot the image. (If yours is somewhere else for some reason, find it and change the path below accordingly.)

Assuming that you already have created a virtual machine called HASS (using the import image button in Cockpit and using the QCOW2 image provided on the home assistant installation page, you can now edit that VM on the command line (make sure the machine is turned off before proceeding):

virsh edit HASS

Locate the section, mine looked like this:

   <os>
     <type arch='x86_64' machine='pc-q35-3.1'>hvm</type>
     <boot dev='hd'/>
   </os>

And insert <loader readonly='yes' type='rom'>/usr/share/ovmf/OVMF.fd</loader> after the type tag:

   <os>
     <type arch='x86_64' machine='pc-q35-3.1'>hvm</type>
     <loader readonly='yes' type='rom'>/usr/share/ovmf/OVMF.fd</loader>
     <boot dev='hd'/>
   </os>

Save and exit and you're done. If you now start VM it should boot the HASS image.