CA-411122: do not call set-iscsi-initiator with an empty string for IQN (#6474)

Back in 2018 a4a94b3 rejected empty
IQNs in set_iscsi_iqn API calls. However hosts are created with an empty
IQN, and if this code runs too early then it will attempt to call
`set-iscsi-initiator` with an empty string for the IQN:
```
/opt/xensource/libexec/set-iscsi-initiator  myhost
```

About a second later the script is called again with the correct value.
This could potentially result in the iscsid service being restarted
multiple times (and if a restart is still pending when restart is called
a 2nd time I'm not sure it'll take effect, so we might be left with an
empty initiator, I have also seen a GFS2 SR plug failure following
this).

It is best to avoid setting empty initiators. The exception would be
raised and ignore due to the log_and_ignore in the caller.

Also log wherever the IQN is set using %S, so that we notice if it ends
up containing some extra whitespace characters.

Author: edwintorok

ocaml/xapi/xapi_host.ml

@@ -2790,6 +2790,7 @@ let set_uefi_certificates ~__context ~host:_ ~value:_ =
 let set_iscsi_iqn ~__context ~host ~value =
   if value = "" then
     raise Api_errors.(Server_error (invalid_value, ["value"; value])) ;
+  D.debug "%s: iqn=%S" __FUNCTION__ value ;
   (* Note, the following sequence is carefully written - see the
      other-config watcher thread in xapi_host_helpers.ml *)
   Db.Host.remove_from_other_config ~__context ~self:host ~key:"iscsi_iqn" ;

ocaml/xapi/xapi_host_helpers.ml

@@ -497,10 +497,13 @@ module Configuration = struct
     [iqn; hostname_chopped]
 
   let set_initiator_name iqn =
+    if iqn = "" then
+      raise Api_errors.(Server_error (invalid_value, ["iqn"; iqn])) ;
     let hostname = Unix.gethostname () in
     (* CA-377454 - robustness, create dir if necessary *)
     Unixext.mkdir_rec "/var/lock/sm/iscsiadm" 0o700 ;
     let args = make_set_initiator_args iqn hostname in
+    D.debug "%s: iqn=%S" __FUNCTION__ iqn ;
     ignore (Helpers.call_script !Xapi_globs.set_iSCSI_initiator_script args)
 
   let set_multipathing enabled =
@@ -541,6 +544,7 @@ module Configuration = struct
             | Some "" ->
                 ()
             | Some iqn when iqn <> host_rec.API.host_iscsi_iqn ->
+                D.debug "%s: iqn=%S" __FUNCTION__ iqn ;
                 Client.Client.Host.set_iscsi_iqn ~rpc ~session_id ~host:host_ref
                   ~value:iqn
             | _ ->