Make sure that request URI actually _ends_ with /sitemap.xml
Hook to ProcessPageView::pageNotFound instead of Page::render

In that rare (but possible) situation where user has created a page
called "sitemap.xml", show that instead of dynamic sitemap.
Limit sitemap.xml to URLs in same directory or below it

Protocol specification at sitemaps.org says that sitemap.xml should
only contain pages in or below it's root directory in page tree.
Add support for Multisite module