x86/xen: add reschedule point when mapping foreign GFNs

Mapping a large range of foreign GFNs can take a long time, add a
reschedule point after each batch of 16 GFNs.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>