os: add 'Xml_node::for_each_sub_node'

The new function template simplifies the common case of iterating
through the sub nodes of an XML node.

repos/os/include/util/xml_node.h

@@ -656,6 +656,35 @@ namespace Genode {
 				throw Nonexistent_sub_node();
 			}
 
+			/**
+			 * Execute functor 'fn' for each sub node of specified type
+			 */
+			template <typename FN>
+			void for_each_sub_node(char const *type, FN const &fn)
+			{
+				if (_num_sub_nodes == 0)
+					return;
+
+				Xml_node node = sub_node();
+				for (int i = 0; ; node = node.next()) {
+
+					if (!type || node.has_type(type))
+						fn(node);
+
+					if (++i == _num_sub_nodes)
+						break;
+				}
+			}
+
+			/**
+			 * Execute functor 'fn' for each sub node
+			 */
+			template <typename FN>
+			void for_each_sub_node(FN const &fn)
+			{
+				for_each_sub_node(nullptr, fn);
+			}
+
 			/**
 			 * Return Nth attribute of XML node
 			 *