xen: make sure xen_extra_mem_start is beyond all non-RAM e820

If Xen gives us non-RAM E820 entries (dom0 only, typically), then
make sure the extra RAM region is beyond them.  It's OK for
the extra space to grow into E820 regions, however.

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