validate nodeselector after store is loaded

this shows the validation status immediatly if no node is selected (for
instance because no node is online or you have only one node and open
the migration window)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

www/manager6/form/NodeSelector.js

@@ -112,5 +112,8 @@ Ext.define('PVE.form.NodeSelector', {
 	    }
 	}));
 
+	me.mon(me.getStore(), 'load', function(){
+	    me.isValid();
+	});
     }
 });