tools/virtio: remove stray characters

__read_once_size() is not a macro, remove those '/'s.

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Message-Id: <20221128034347.990-2-dave@stgolabs.net>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>