XSI-2128: Ignore RBAC when destroying internal tasks (#6885)

When calling `VDI.copy` or `VDI.pool_migrate` with `vm_power_admin`
role, xapi may forward the operation to a remote host. In this case,
xapi creates a pool session on the remote host and create a new task.
When the operation completes, `try_internal_async` uses the user's
session to destroy the task that was created by an internal pool
session, but the user doesn’t have the permission to destory other
user’s task (task.destroy/any), so it fails.

Solution:
This is an internal cleanup operation, so it doesn’t need user RBAC
restriction and checking. Ignore RBAC when destroying internal tasks by
calling Db_actions.DB_Action.Task.destroy directly.

Author: robhoes

ocaml/xapi/helpers.ml

@@ -2025,7 +2025,7 @@ let try_internal_async ~__context (marshaller : Rpc.t -> 'b)
         )
         (fun () ->
           info "try_internal_async: destroying task: t = ( %s )" ref ;
-          TaskHelper.destroy ~__context t
+          Db.Task.destroy ~__context ~self:t
         )
 
 module PoolSecret : sig