OCaml5: Annotate non-returning functions with types (#6081)

These become warnings in ocaml 5.0+

ocaml/xapi/xapi.ml

@@ -180,7 +180,7 @@ let init_args () =
     Xapi_globs.xenopsd_queues := ["xenopsd"]
   )
 
-let wait_to_die () =
+let wait_to_die () : unit =
   (* don't call Thread.join cos this interacts strangely with OCAML runtime and stops
      the OCAML-level signal handlers ever getting called... Thread.delay is fine tho' *)
   while true do

ocaml/xapi/xapi_ha.ml

@@ -130,7 +130,7 @@ let uuid_of_host_address address =
 let on_master_failure () =
   (* The plan is: keep asking if I should be the master. If I'm rejected then query the
      live set and see if someone else has been marked as master, if so become a slave of them. *)
-  let become_master () =
+  let become_master () : unit =
     info "This node will become the master" ;
     Xapi_pool_transition.become_master () ;
     info "Waiting for server restart" ;

ocaml/xenopsd/cli/xn.ml

@@ -1051,7 +1051,7 @@ let unix_proxy path =
   | 0 ->
       let buf = Bytes.make 16384 '\000' in
       let accept, _ = Unix.accept listen in
-      let copy a b =
+      let copy a b : unit =
         while true do
           let n = Unix.read a buf 0 (Bytes.length buf) in
           if n = 0 then exit 0 ;