xen/ioreq: Do not let bufioreq to be used on other than x86 arches

xen/ioreq: Do not let bufioreq to be used on other than x86 arches

This patch prevents the device model running on other than x86
systems to use buffered I/O feature for now.

Please note, there is no caller which requires to send buffered
I/O request on Arm currently and the purpose of this check is
to catch any future user of bufioreq.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Acked-by: Paul Durrant <paul@xen.org>