virtio: Extend the admin command to include the result size

Extend the admin command by incorporating a result size field.

This allows higher layers to determine the actual result size from the
backend when this information is not included in the result_sg.

The additional information introduced here will be used in subsequent
patches of this series.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20241113115200.209269-3-yishaih@nvidia.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>