Comment 13 for bug 1815889

That left only the actual disk+iso of fedora vs ubuntu cloudimg based qcow and that the boxes VM used userspace networking. Still the issue remained.

But I realized there is one more difference, the Boxes VM runs in user context while mine is a system level VM (qemu:///system) running the gl essentially headless until one connects to the local spice port.
But the gnome boxes VM was having the UI up immediately connecting to it once available.

So I defined the XML of the gnome-boxes VM in my qemu:///system libvirt context.
This - as expected (I copied the files to /var/lib/libvirt/images and adapted the paths).
This makes it work which is at least some lead to follow.