synced_folders/nfs: allow plugins to provide valid NFS ids

plugins/synced_folders/nfs/synced_folder.rb

@@ -52,10 +52,13 @@ module VagrantPlugins
       end
 
       def cleanup(machine, opts)
+        ids = opts[:nfs_valid_ids]
+        if !ids
           # Get the ID of all active machines.
           ids = machine.env.active_machines.map do |name, provider|
             machine.env.machine(name, provider).id
           end
+        end
 
         # Prune any of the unused machines
         machine.env.host.nfs_prune(ids)