reset FileSelector grid when changing the storage

otherwise, when the new storage returns an error, we show the content
of the old storage

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

www/manager6/form/FileSelector.js

@@ -34,6 +34,7 @@ Ext.define('PVE.form.FileSelector', {
 	    url: url
 	});
 
+	me.store.removeAll();
 	me.store.load();
     },