Code review comment for lp:~qzhang/lava-dispatcher/test-abrek

Finish on most comments.

But I can't umount /mnt/root/dev after I use "mount --rbind /dev /mnt/root/dev", however, the mount occurs in master image, so I don't think it does matter, or who know how to umount rebind device?

root@master:~# umount -f /mnt/root/dev
umount2: Device or resource busy
umount: /mnt/root/dev: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))