xen/ioreq: Move x86's io_completion/io_req fields to struct vcpu

xen/ioreq: Move x86's io_completion/io_req fields to struct vcpu

The IOREQ is a common feature now and these fields will be used
on Arm as is. Move them to common struct vcpu as a part of new
struct vcpu_io and drop duplicating "io" prefixes. Also move
enum hvm_io_completion to xen/sched.h and remove "hvm" prefixes.

This patch completely removes layering violation in the common code.

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