xen/virtio: Enable restricted memory access using Xen grant mappings

In order to support virtio in Xen guests add a config option XEN_VIRTIO
enabling the user to specify whether in all Xen guests virtio should
be able to access memory via Xen grant mappings only on the host side.

Also set PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS feature from the guest
initialization code on Arm and x86 if CONFIG_XEN_VIRTIO is enabled.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/1654197833-25362-5-git-send-email-olekstysh@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>