CA-407033: Call receive_finalize2 synchronously (#6312)

`Remote.receive_finalize2` is called at the end of SXM to clean things
up and compose the base and leaf images together. The compose operation
should only be called while the VDI is deactivated. Currently a thread
is created to call `receive_finalize2`, which could caused problems
where the VM itself gets started while the
`receive_finalize2`/`VDI.compose` is still in progress. This is not a
safe operation to do.

The fix here is simply to wait for the thread to finish.

Author: Vincent-lau

ocaml/xapi/storage_migrate.ml

@@ -1338,22 +1338,14 @@ let post_deactivate_hook ~sr ~vdi ~dp:_ =
              Storage_utils.rpc ~srcstr:"smapiv2" ~dststr:"dst_smapiv2"
                remote_url
          end)) in
-         let t =
-           Thread.create
-             (fun () ->
-               debug "Calling receive_finalize2" ;
-               log_and_ignore_exn (fun () ->
-                   Remote.DATA.MIRROR.receive_finalize2 "Mirror-cleanup" id
-               ) ;
-               debug "Finished calling receive_finalize2" ;
-               State.remove_local_mirror id ;
-               debug "Removed active local mirror: %s" id
-             )
-             ()
-         in
-         Option.iter (fun id -> Scheduler.cancel scheduler id) r.watchdog ;
-         debug "Created thread %d to call receive finalize and dp destroy"
-           (Thread.id t)
+         debug "Calling receive_finalize2" ;
+         log_and_ignore_exn (fun () ->
+             Remote.DATA.MIRROR.receive_finalize2 "Mirror-cleanup" id
+         ) ;
+         debug "Finished calling receive_finalize2" ;
+         State.remove_local_mirror id ;
+         debug "Removed active local mirror: %s" id ;
+         Option.iter (fun id -> Scheduler.cancel scheduler id) r.watchdog
      )
 
 let nbd_handler req s ?(vm = "0") sr vdi dp =