Revert VM migration shutdown order change + change post_detach to post_deactivate (#6260)

There are two parts to this PR:

1. Reverts the VM migration ordering change, which was previously
thought to be necessary as I did not see `VM_save` would actually
deactivate the source VM datapath. Thanks @edwintorok for pointing that
out.
2. Change `post_detach_hook` to `post_deactivate_hook`, as this should
have been called as soon as the datapath is deactivated, at which point
all r/w using that datapath will be stopped.

More details in the commit message.

Author: robhoes

ocaml/xapi/storage_migrate.ml

@@ -1320,7 +1320,7 @@ let pre_deactivate_hook ~dbg:_ ~dp:_ ~sr ~vdi =
              s.failed <- true
      )
 
-let post_detach_hook ~sr ~vdi ~dp:_ =
+let post_deactivate_hook ~sr ~vdi ~dp:_ =
   let open State.Send_state in
   let id = State.mirror_id_of (sr, vdi) in
   State.find_active_local_mirror id

ocaml/xapi/storage_smapiv1_wrapper.ml

@@ -422,10 +422,10 @@ functor
               | Vdi_automaton.Deactivate ->
                   Storage_migrate.pre_deactivate_hook ~dbg ~dp ~sr ~vdi ;
                   Impl.VDI.deactivate context ~dbg ~dp ~sr ~vdi ~vm ;
+                  Storage_migrate.post_deactivate_hook ~sr ~vdi ~dp ;
                   vdi_t
               | Vdi_automaton.Detach ->
                   Impl.VDI.detach context ~dbg ~dp ~sr ~vdi ~vm ;
-                  Storage_migrate.post_detach_hook ~sr ~vdi ~dp ;
                   vdi_t
             in
             Sr.add_or_replace vdi new_vdi_t sr_t ;

ocaml/xenopsd/lib/xenops_server.ml

@@ -2732,23 +2732,6 @@ and perform_exn ?result (op : operation) (t : Xenops_task.task_handle) : unit =
             ) ;
             debug "VM.migrate: Synchronisation point 1"
           in
-          let pause_src_vm () =
-            debug
-              "VM.migrate: pause src vm before allowing destination to proceed" ;
-            (* cleanup tmp src VM *)
-            let atomics =
-              [
-                VM_hook_script_stable
-                  ( id
-                  , Xenops_hooks.VM_pre_destroy
-                  , Xenops_hooks.reason__suspend
-                  , new_src_id
-                  )
-              ]
-              @ atomics_of_operation (VM_shutdown (new_src_id, None))
-            in
-            perform_atomics atomics t
-          in
           let final_handshake () =
             Handshake.send vm_fd Handshake.Success ;
             debug "VM.migrate: Synchronisation point 3" ;
@@ -2789,10 +2772,7 @@ and perform_exn ?result (op : operation) (t : Xenops_task.task_handle) : unit =
              the main VM migration sequence. *)
           match VGPU_DB.ids id with
           | [] ->
-              first_handshake () ;
-              save () ;
-              pause_src_vm () ;
-              final_handshake ()
+              first_handshake () ; save () ; final_handshake ()
           | (_vm_id, dev_id) :: _ ->
               let url =
                 make_url "/migrate/vgpu/"
@@ -2809,12 +2789,20 @@ and perform_exn ?result (op : operation) (t : Xenops_task.task_handle) : unit =
                   first_handshake () ;
                   save ~vgpu_fd:(FD vgpu_fd) ()
               ) ;
-              pause_src_vm () ;
               final_handshake ()
       ) ;
-      let cleanup_src_vm () =
-        let atomics =
-          [
+      (* cleanup tmp src VM *)
+      let atomics =
+        [
+          VM_hook_script_stable
+            ( id
+            , Xenops_hooks.VM_pre_destroy
+            , Xenops_hooks.reason__suspend
+            , new_src_id
+            )
+        ]
+        @ atomics_of_operation (VM_shutdown (new_src_id, None))
+        @ [
             VM_hook_script_stable
               ( id
               , Xenops_hooks.VM_post_destroy
@@ -2823,10 +2811,8 @@ and perform_exn ?result (op : operation) (t : Xenops_task.task_handle) : unit =
               )
           ; VM_remove new_src_id
           ]
-        in
-        perform_atomics atomics t
       in
-      cleanup_src_vm ()
+      perform_atomics atomics t
   | VM_receive_memory
       {
         vmr_id= id