daemon: Reimplement ‘device_index’ API in OCaml.

daemon/devsparts.ml

@@ -113,3 +113,14 @@ let is_whole_device device =
 
   try ignore (stat devpath); true
   with Unix_error ((ENOENT|ENOTDIR), _, _) -> false
+
+let device_index device =
+  (* This is the algorithm which was used by the C version.  Why
+   * can't we use drive_index from C_utils?  XXX
+   *)
+  let rec loop i = function
+    | [] -> failwithf "%s: device not found" device
+    | dev :: devices when dev = device -> i
+    | _ :: devices -> loop (i+1) devices
+  in
+  loop 0 (list_devices ())