qapi2texi works with schema expression trees.  Such a tight coupling
to schema language syntax is not a good idea.  Convert it to the visitor
interface the other generators use.

No change to generated documentation.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-17-git-send-email-armbru@redhat.com>