virt tools: Use Sys.file_exists instead of clumsy open_in/close_in hack.

No functional change.

Fixes commit c466d4cb92.

customize/firstboot.ml

@@ -193,13 +193,7 @@ module Windows = struct
     let srvany =
       try
         List.find (
-          fun service ->
-            try
-              let chan = open_in (virt_tools_data_dir // service) in
-              close_in chan;
-              true
-            with _ ->
-              false
+          fun service -> Sys.file_exists (virt_tools_data_dir // service)
         ) services
       with Not_found ->
        error (f_"One of rhsrvany.exe or pvvxsvc.exe is missing in %s.  One of them is required in order to install Windows firstboot scripts.  You can get one by building rhsrvany (https://github.com/rwmjones/rhsrvany)")

v2v/convert_windows.ml

@@ -48,14 +48,7 @@ let convert ~keep_serial_console (g : G.guestfs) inspect source rcaps =
   let installer =
     try
       let t, tool = List.find (
-        fun (_, tool) ->
-          try (
-            let exe_path = virt_tools_data_dir // tool in
-            let chan = open_in exe_path in
-            close_in chan;
-            true
-          ) with _ ->
-            false
+        fun (_, tool) -> Sys.file_exists (virt_tools_data_dir // tool)
       ) tools in
       Some (t, virt_tools_data_dir // tool)
     with Not_found -> (