Fix rename for file-backed instances

Currently the code wrongly changes the disk logical/physical id
component representing the path from "$storage_dir/$iname/disk$seq" to
"$storage_dir/$iname/disk/$seq" (note the additional slash) breaking the
rename.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>