convert map_domain_page() to use the new mfn_t type

convert map_domain_page() to use the new mfn_t type

Reworked the internals and declaration, applying (un)boxing
where needed. Converted calls to map_domain_page() to
provide mfn_t types, boxing where needed.

Signed-off-by: Ben Catterall <Ben.Catterall@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>