hw/xen: xenpvh: Disable buffered IOREQs for ARM

hw/xen: xenpvh: Disable buffered IOREQs for ARM

Add a way to enable/disable buffered IOREQs for PVH machines
and disable them for ARM. ARM does not support buffered
IOREQ's nor the legacy way to map IOREQ info pages.

See the following for more details:
https://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=2fbd7e609e1803ac5e5c26e22aa8e4b5a6cddbb1
https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/arm/ioreq.c;h=2e829d2e7f3760401b96fa7c930e2015fb1cf463;hb=HEAD#l138

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@amd.com>