Integrate with /var/lib/machines/

[spanezz]

systemd-nspawn has some infrastructure to manage OS images under /var/lib/machines/ (see for example machinectl pull-tar).

We might be able to integrate with it as a storage of OS images, or follow the same structure when OS images are stored in the user's home directory, or in a file

[spanezz]

Meanwhile I added support for using .tar.xz as precached distribution images, so that we are compatible with tarballs in nspawn.org, for example: https://hub.nspawn.org/storage/centos/7/tar/

It is possible to verify signatures on those images, and an implementation of it can be found at https://github.com/nspawn/nspawn/blob/master/nspawn

I still cannot figure out if we can just converge with /var/lib/machines/.

We have extra features over that, like maintaining the images directory on a btrfs partition on a file, or a per-user imagedir, but we could still use /var/lib/machines as systemd uses it, as an alternative images backend, perhaps even as the default one if imagedir is not configured

[spanezz]

I now changed default imagedir to /var/lib/machines, and images bootstrapped by moncic-ci can be started out of the box with machinectl start.

I'm closing this ticket, which is quite generic and sort of addressed by this change, and opening a new one ot integrate with nspawn.org and machinectl pull-tar