xen: don't add extra_pages for RAM after mem_end

If an E820 region is entirely beyond mem_end, don't attempt to truncate
it and add the truncated pages to extra_pages, as they will be negative.

Also, make sure the extra memory region starts after all BIOS provided
E820 regions (and in the case of RAM regions, post-clipping).

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>