Report host and cell as stored in disk.

Reporting the host and cell makes easier to fire another host, hail and join the cohort or simply migrate the disk.
This isn't the right place to log "read superblock...". This is logging that message every time it constructs a superblock object, including when it constructs one to be written. In other words, when a checkpoint finishes up and writes the new superblock, it logs a message "read superblock...".