Add root_element awareness to wlxml

The Wlxml module was not able to support types that were the root
element of their respective document.

This commit added initial root_element awareness in two forms. First is
a 'root_element' class function (with 'root_element?' counterpart) that
designates the provider as representing a root_element. Second was a
modification to how discovered instances and resources are matched.

More needs to be done along the lines of supporting root_elements. There
are a few other sections that were designed without taking them into
account.