fake a dom0 json config in 4.5 unstable

In Centos 7 with systemd:

sudo -s
cd /var/lib/xen
cat > userdata-d.0.00000000-0000-0000-0000-000000000000.libxl-json<<EOL
[
    {
        "domid": 0,
        "config": {
            "c_info": {
                "name": "Domain-0"
            },
            "b_info": {
                "max_memkb": 622592,
                "target_memkb": 524287,
                "sched_params": {

                },
                "type.invalid": {

                }
            }
        }
    }
]
EOL

test:
xl list -l 0