qapi-types: add #if conditions to types & visitors

Types & visitors are coupled and must be handled together to avoid
temporary build regression.

Wrap generated types/visitor code with #if/#endif using the context
helpers. Derived from a patch by Marc-André.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180703155648.11933-12-marcandre.lureau@redhat.com>