Tweaks of reboot-related logging

[happz]

• core code should mention it is now going to trigger a reboot and with what action;
• artemis was duplicating output and deciding the reboot command, that is guaranteed by Guest.reboot() already;
• virtual did not log its soft-reboot code path;
• dropping the increased verbosity in /tests/execute/nonroot that was needed to debug bootc vs VM reboot.

Pull Request Checklist

• [x] implement the feature