xen: move remaining mmu-related stuff into mmu.c

Impact: Cleanup

Move remaining mmu-related stuff into mmu.c.
A general cleanup, and lay the groundwork for later patches.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>