x86-64: use MFNs for linking together pages on lists

x86-64: use MFNs for linking together pages on lists

Unless more than 16Tb are going to ever be supported in Xen, this will
allow reducing the linked list entries in struct page_info from 16 to
8 bytes.

This doesn't modify struct shadow_page_info, yet, so in order to meet
the constraints of that 'mirror' structure the list entry gets
artificially forced to be 16 bytes in size. That workaround will be
removed in a subsequent patch.

Signed-off-by: Jan Beulich <jbeulich@novell.com>