XSI-1954: Only block pool join for clustering on non-management VLAN (#6585)

It is because the known issue is only with non-management VLAN:

https://docs.xenserver.com/en-us/xenserver/8/storage/format#thin-provisioned-shared-gfs2-block-storage

We recommend not to use a GFS2 SR with a VLAN due to a known issue where
you cannot add or remove hosts on a clustered pool if the cluster
network is on a non-management VLAN.

Author: gangj

ocaml/xapi/xapi_pool.ml

@@ -118,17 +118,23 @@ let pre_join_checks ~__context ~rpc ~session_id ~force =
       | pif when pif = Ref.null ->
           ()
       | pif -> (
-        match Client.PIF.get_VLAN ~rpc ~session_id ~self:pif with
-        | vlan when vlan > 0L ->
-            error "Cannot join pool whose clustering is enabled on VLAN network" ;
+        match
+          ( Client.PIF.get_VLAN ~rpc ~session_id ~self:pif
+          , Client.PIF.get_management ~rpc ~session_id ~self:pif
+          )
+        with
+        | vlan, false when vlan > 0L ->
+            error
+              "Cannot join pool whose clustering is enabled on a \
+               non-management VLAN network" ;
             raise
               (Api_errors.Server_error
                  ( Api_errors
                    .pool_joining_pool_cannot_enable_clustering_on_vlan_network
                  , [Int64.to_string vlan]
                  )
               )
-        | 0L | _ -> (
+        | _ -> (
             let clustering_bridges_in_pool =
               ( match
                   Client.PIF.get_bond_master_of ~rpc ~session_id ~self:pif