xen/mm: Switch map_pages_to_xen to use MFN typesafe

xen/mm: Switch map_pages_to_xen to use MFN typesafe

The current prototype is slightly confusing because it takes a virtual
address and a physical frame (not address!). Switching to MFN will improve
safety and reduce the chance to mistakenly invert the 2 parameters.

Also, take the opportunity to switch (a - b) >> PAGE_SHIFT to
PFN_DOWN(a - b) in the code modified.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>