move XENMAPSPACE_grant_table code into grant_table.c

move XENMAPSPACE_grant_table code into grant_table.c

The x86 and arm versions of XENMAPSPACE_grant_table handling are nearly
identical. Move the code into a function in grant_table.c and add an
architecture dependant hook to handle the differences.

Switch to mfn_t in order to be more type safe.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>