vhd-tool: when opening raw images, peek behind any tapdisks

When opening a file for reading, if we open a tapdisk device
then `SEEK_DATA` and `SEEK_HOLE` won't work. Instead if we peek
behind the tapdisk then the filesystem can implement `SEEK_*`
(or not).

Signed-off-by: David Scott <dave.scott@citrix.com>

_oasis

@@ -16,7 +16,7 @@ Executable "vhd-tool"
   MainIs:             main.ml
   Custom:             true
   Install:            false
-  BuildDepends:       lwt, lwt.unix, lwt.syntax, lwt.preemptive, threads, vhd-format, vhd-format.lwt, cmdliner, nbd, nbd.lwt, uri, cohttp (>= 0.12.0), cohttp.lwt, tar, sha, sha.sha1, io-page.unix, re.str
+  BuildDepends:       lwt, lwt.unix, lwt.syntax, lwt.preemptive, threads, vhd-format, vhd-format.lwt, cmdliner, nbd, nbd.lwt, uri, cohttp (>= 0.12.0), cohttp.lwt, tar, sha, sha.sha1, io-page.unix, threads, tapctl, re.str
   CSources:           sendfile64_stubs.c
 
 Executable "sparse_dd"

src/image.ml

@@ -1,5 +1,3 @@
-open Xenstore
-
 let (|>) a b = b a
 module Opt = struct
 	let default d = function

src/impl.ml

@@ -651,9 +651,15 @@ let make_stream common source relative_to source_format destination_format =
     ( match relative_to with None -> return None | Some f -> Vhd_IO.openchain ~path f false >>= fun t -> return (Some t) ) >>= fun from ->
     Vhd_input.raw ?from t
   | "raw", "vhd" ->
+    let source = match Image.of_device source with
+      | Some (`Raw x) -> x (* bypass any tapdisk and use the raw file *)
+      | _ -> source in
     Raw_IO.openfile source false >>= fun t ->
     Raw_input.vhd t
   | "raw", "raw" ->
+    let source = match Image.of_device source with
+      | Some (`Raw x) -> x (* bypass any tapdisk and use the raw file *)
+      | _ -> source in
     Raw_IO.openfile source false >>= fun t ->
     Raw_input.raw t
   | _, _ -> assert false