Gets the extension block for the relevant type
XmlDocument with just the correct block in it
Takes XML of the form:
<AnyName>
<extension type="validation"> ... contents ... </extension>
<extension type="otherInformation"> ... contents ... </extension>
<extension type="foobar"> ... contents ... </extension>
</AnyName>
And returns:
<extension type="validation"> ... contents ... </extension>
Also handles single extensions in the form of:
<extension type="validation"> ... contents ... </extension>