move XML loading and saving into XmlFileUtil

stepmania/src/Background.cpp

@@ -774,7 +774,7 @@ void BackgroundImpl::Layer::UpdateCurBGChange( const Song *pSong, float fLastMus
 		if( iter == m_BGAnimations.end() )
 		{
 			XNode *pNode = change.m_def.CreateNode();
-			LOG->Warn( "Tried to switch to a background that was never loaded\n" + pNode->GetXML() );
+			LOG->Warn( "Tried to switch to a background that was never loaded\n" + XmlFileUtil::GetXML(pNode) );
 			SAFE_DELETE( pNode );
 			return;
 		}

stepmania/src/Bookkeeper.cpp

@@ -138,7 +138,7 @@ void Bookkeeper::WriteToDisk()
 	}
 
 	XNode *xml = CreateNode();
-	xml->SaveToFile( f );
+	XmlFileUtil::SaveToFile( xml, f );
 	delete xml;
 }