Support diskless directly with bootc

[cgwalters]

Chatting with someone here at KubeCon, we realized a nice way to diskless would be to use PXE boot, have bootc in the initramfs, then fetch the image to RAM and pivot into it.

This would replace squashfs with OCI which would be pretty cool.

[kfox1111]

This would be great. :)

[kfox1111]

It could potentially be used as a base implementation for kubevirt/kubevirt#13212.

[scottcwang]

Dracut dmsquash-live and livenet currently support root=live:LABEL=*, root=live:UUID=*, root=live:PARTLABEL=*, root=live:PARTUUID=*, root=live:CDLABEL=*, root=live:nfs*, root=live:http://*, root=live:https://*, root=live:ftp://*, root=live:torrent://*, root=live:tftp://*. Perhaps some of this existing logic could get reused to parse the root dracut argument, find the container image, and pull it into the same RAM disk that dracut already uses for live booting.