qapi: Make code-generating visitors use QAPIGen more

The use of QAPIGen is rather shallow so far: most of the output
accumulation is not converted.  Take the next step: convert output
accumulation in the code-generating visitor classes.  Helper functions
outside these classes are not converted.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180211093607.27351-20-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
[eblake: rebase to earlier guardstart cleanup]
Signed-off-by: Eric Blake <eblake@redhat.com>