CP-50836: Add VM_migrate_downtime and request_shutdown spans

This simplifies making optimisations as it saves having to look in the logs for
these events.

Signed-off-by: Steven Woods <steven.woods@cloud.com>

Author: snwoods

ocaml/xenopsd/xc/xenops_server_xen.ml

@@ -1778,7 +1778,8 @@ module VM = struct
         if di.Xenctrl.total_memory_pages = 0n then
           raise (Xenopsd_error Domain_not_built) ;
         Domain.unpause ~xc di.Xenctrl.domid
-    )
+    ) ;
+    with_tracing ~task:t ~name:"VM_migrate_downtime_end" Fun.id
 
   let set_xsdata task vm xsdata =
     on_domain task vm (fun _ xs _ _ di ->
@@ -2515,6 +2516,7 @@ module VM = struct
                 @@ fun () -> pre_suspend_callback task
                 ) ;
 
+                with_tracing ~task ~name:"VM_save_request_shutdown" @@ fun () ->
                 if
                   not
                     ( with_tracing ~task
@@ -2523,6 +2525,7 @@ module VM = struct
                     )
                 then
                   raise (Xenopsd_error Failed_to_acknowledge_suspend_request) ;
+                with_tracing ~task ~name:"VM_migrate_downtime_begin" Fun.id ;
                 if
                   not
                     ( with_tracing ~task