CA-398128: Be wary that dates in database lose precision (#6004)

Dates converted from unix timestamps have subsecond precision, but dates
serialized in the database do not. This is a change in behaviour
introduced in 233b96b

This fix was supposed to be included in the change, but got lost. No
other users were found that hit this change in behaviour.

Fixes the test about VMs pending guidances being removed when not
needed, job 411209

ocaml/xapi/xapi_xenops.ml

@@ -2282,13 +2282,15 @@ let update_vm ~__context id =
                 Option.iter
                   (fun (_, state) ->
                     let metrics = Db.VM.get_metrics ~__context ~self in
+                    (* Clamp time to full seconds, stored timestamps do not
+                        have decimals *)
                     let start_time =
-                      Date.of_unix_time state.Vm.last_start_time
+                      Float.floor state.Vm.last_start_time |> Date.of_unix_time
                     in
-                    if
-                      start_time
-                      <> Db.VM_metrics.get_start_time ~__context ~self:metrics
-                    then (
+                    let expected_time =
+                      Db.VM_metrics.get_start_time ~__context ~self:metrics
+                    in
+                    if Date.is_later ~than:expected_time start_time then (
                       debug
                         "xenopsd event: Updating VM %s last_start_time <- %s" id
                         Date.(to_rfc3339 (of_unix_time state.Vm.last_start_time)) ;