virtio_config: clarify output parameters

This was ambiguous enough for a broken patch (206cc445 ("virtio:
reject shm region if length is zero")) to make it into the kernel, so
make it clearer.

Link: https://lore.kernel.org/r/20250816071600-mutt-send-email-mst@kernel.org/


Signed-off-by: Alyssa Ross <hi@alyssa.is>
Message-Id: <20250829150944.233505-1-hi@alyssa.is>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>