CP-50836: Add VM_migrate_downtime and request_shutdown spans

This simplifies making optimisations as it removes the need to look in
the logs for these events.

Signed-off-by: Steven Woods <[email protected]>

Author: snwoods

ocaml/xenopsd/lib/xenops_server.ml

@@ -2988,6 +2988,7 @@ and perform_exn ?result (op : operation) (t : Xenops_task.task_handle) : unit =
             ]
           )
           t ;
+        with_tracing ~task:t ~name:"VM_migrate_downtime_end" Fun.id ;
         Handshake.send s Handshake.Success ;
         debug "VM.receive_memory: Synchronisation point 4"
       with e ->

ocaml/xenopsd/xc/xenops_server_xen.ml

@@ -2515,6 +2515,7 @@ module VM = struct
                 @@ fun () -> pre_suspend_callback task
                 ) ;
 
+                with_tracing ~task ~name:"VM_save_request_shutdown" @@ fun () ->
                 if
                   not
                     ( with_tracing ~task
@@ -2523,6 +2524,13 @@ module VM = struct
                     )
                 then
                   raise (Xenopsd_error Failed_to_acknowledge_suspend_request) ;
+                (* If this is for a migration, record the begin time *)
+                ( match data with
+                | FD _ ->
+                    with_tracing ~task ~name:"VM_migrate_downtime_begin" Fun.id
+                | _ ->
+                    ()
+                ) ;
                 if
                   not
                     ( with_tracing ~task