xen/ioreq: Move x86's ioreq_server to struct domain

xen/ioreq: Move x86's ioreq_server to struct domain

The IOREQ is a common feature now and this struct will be used
on Arm as is. Move it to common struct domain. This also
significantly reduces the layering violation in the common code
(*arch.hvm* usage).

We don't move ioreq_gfn since it is not used in the common code
(the "legacy" mechanism is x86 specific).

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
CC: Julien Grall <julien.grall@arm.com>
[On Arm only]
Tested-by: Wei Chen <Wei.Chen@arm.com>