docs: driver-api: virtio: commentize spec version checking

A sentence that checks for later spec version is meant for developers
hacking the documentation source. Make it comment block (hidden from
actual output).

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Message-Id: <20221220095828.27588-4-bagasdotme@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>