ext5migrate: replace call to private ExtJS function getRoot()

getRoot() semantics changed with ExtJS5.

To avoid further annoyances when ExtJS6 is around the corner, replace the call
to this private method, set rootProperty in the ObjectStore call,
and use the value of rootProperty to filter out the content of the server
answer.

www/manager5/data/ObjectStore.js

@@ -24,7 +24,8 @@ Ext.define('PVE.data.ObjectStore',  {
                 reader: {
 		    type: 'jsonobject',
 		    rows: config.rows,
-		    readArray: config.readArray
+		    readArray: config.readArray,
+		    rootProperty: config.root || 'data'
 		}
             }
         });

www/manager5/data/reader/JsonObject.js

@@ -31,8 +31,6 @@ Ext.define('PVE.data.reader.JsonObject', {
     extend: 'Ext.data.reader.Json',
     alias : 'reader.jsonobject',
     
-    root: 'data',
-  
     readArray: false,
 
     rows: undefined,
@@ -50,8 +48,9 @@ Ext.define('PVE.data.reader.JsonObject', {
 
 	var data = [];
         try {
-            var result = Ext.decode(response.responseText);
-	    var root = me.getRoot(result);
+        var result = Ext.decode(response.responseText);
+        // get our data items inside the server response
+        var root = result[me.getRootProperty()];
 
 	    if (me.readArray) {