qapi: New QAPISchemaVisitor

The visitor will help keeping the code generation code simple and
reasonably separated from QAPISchema details.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1442401589-24189-5-git-send-email-armbru@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>