xen: clean up "extra" memory handling some more

Make sure that extra_pages is added for all E820_RAM regions beyond
mem_end - completely excluded regions as well as the remains of partially
included regions.

Also makes sure the extra region is not unnecessarily high, and simplifies
the logic to decide which regions should be added.

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