Fedora CoreOS can be provisioned on a QEMU virtual machine by providing
the Ignition configuration via `fw_cfg` value. Unfortunately, the
`string` method does not work with JSON values, so we have to use
`file`. The configuration file has to be uploaded via SFTP, rather than
`virsh vol-import`, since the latter would create the file with the
wrong permissions, and QEMU does not automatically adjust the
permissions of files used this way (like it does for disks).
