Just for the sake of completion. If someone like to use the Gnome Boxes Flatpak, you also need to install kvm-host and add your user to the kvm group like so:
$ sudo swupd bundle-add kvm-host $ sudo usermod -a -G kvm `whoami`
You can type in your username directly if you don’t want to use
If you need USB redirection and file share, use the bundle (
sudo swupd bundle-add gnome-boxes).