mllib: move is_regular_file there

Move the is_regular_file function, implemented in virt-dib and virt-v2v,
to Common_utils.

dib/elements.ml

@@ -147,9 +147,6 @@ let copy_element element destdir blacklist =
   let dirs, nondirs = List.partition is_directory entries in
   let dirs = List.map (fun x -> (x, element.directory // x, destdir // x)) dirs in
   let nondirs = List.map (fun x -> element.directory // x) nondirs in
-  let is_regular_file file =
-    try (Unix.stat file).Unix.st_kind = Unix.S_REG
-    with Unix.Unix_error _ -> false in
   List.iter (
     fun (e, path, destpath) ->
       do_mkdir destpath;

mllib/common_utils.ml

@@ -806,3 +806,7 @@ let read_first_line_from_file filename =
   let line = input_line chan in
   close_in chan;
   line
+
+let is_regular_file path = (* NB: follows symlinks. *)
+  try (Unix.stat path).Unix.st_kind = Unix.S_REG
+  with Unix.Unix_error _ -> false

mllib/common_utils.mli

@@ -248,3 +248,6 @@ val last_part_of : string -> char -> string option
 val read_first_line_from_file : string -> string
 (** Read only the first line (i.e. until the first newline character)
     of a file. *)
+
+val is_regular_file : string -> bool
+(** Checks whether the file is a regular file. *)

v2v/utils.ml

@@ -159,10 +159,6 @@ let find_uefi_firmware guest_arch =
   in
   loop files
 
-let is_regular_file path = (* NB: follows symlinks. *)
-  try (Unix.stat path).Unix.st_kind = Unix.S_REG
-  with Unix.Unix_error _ -> false
-
 (* Given a path of a file relative to the root of the directory tree
  * with virtio-win drivers, figure out if it's suitable for the
   specific Windows flavor of the current guest.