External xml could have empty, one or multiple top-level "Period" elements.
Because xml parser cannot parse the multiple top-level elements
(i.e., no root element), we need to wrap a xml in order to make root element.
See also ISO/IEC 23009-1:2014 5.3.2.2
https://bugzilla.gnome.org/show_bug.cgi?id=774357
