libxc: Fix xc_mem_event.c compilation for ARM

libxc: Fix xc_mem_event.c compilation for ARM

The commit 6ae2df9 "mem_access: Add helper API to setup ring and enable
mem_access¨ break libxc compilation for ARM.

This is because xc_map_foreign_map and xc_domain_decrease_reservation_exact
is taking an xen_pfn_t in parameters. On ARM, xen_pfn_t is always an uin64_t.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Cc: Aravindh Puthiyaparambil <aravindp@cisco.com>
Cc: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>