To set a list of unmanaged-devices you can just do the following.
cat >> /etc/NetworkManager/NetworkManager.conf <<EOF [keyfile] unmanaged-devices=interface-name:vboxnet0;interface-name:virbr0;interface-name:docker0 EOF
and
sudo nmcli connection reload
Strangely I had to put this in NetworkManager.conf. Using
/etc/NetworkManager/conf.d/20-unmanaged-bridges.conf didn’t work.