virtio_blk: fix comment for virtio 1.0

Fix up comment to match virtio 1.0 logic:
virtio_blk_outhdr isn't the first elements anymore,
the only requirement is that it comes first in
the s/g list.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>