-o rhv-upload: Add O_TRUNC to truncate JSON file if it exists

Fixes: #46
Thanks: Nir Soffer

output/output_rhv_upload.ml

@@ -269,7 +269,7 @@ See also the virt-v2v-output-rhv(1) manual.");
          ("rhv_disk_uuids", JSON.List ids) :: json_params
     in
     let precheck_json = dir // "v2vprecheck.json" in
-    let fd = Unix.openfile precheck_json [O_WRONLY; O_CREAT] 0o600 in
+    let fd = Unix.openfile precheck_json [O_WRONLY; O_CREAT; O_TRUNC] 0o600 in
     if Python_script.run_command ~stdout_fd:fd
          precheck_script json_params [] <> 0 then
       error (f_"failed server prechecks, see earlier errors");
@@ -395,7 +395,8 @@ See also the virt-v2v-output-rhv(1) manual.");
 
         (* Start the transfer. *)
         let transfer_json = dir // sprintf "v2vtransfer%d.json" i in
-        let fd = Unix.openfile transfer_json [O_WRONLY; O_CREAT] 0o600 in
+        let fd =
+          Unix.openfile transfer_json [O_WRONLY; O_CREAT; O_TRUNC] 0o600 in
         if Python_script.run_command ~stdout_fd:fd
              transfer_script json_params [] <> 0 then
           error (f_"failed to start transfer, see earlier errors");