qapi: Add #if conditions to generated code members

qapi: Add #if conditions to generated code members

Wrap generated enum and struct members and their supporting code with
#if/#endif, using the .ifcond members added in the previous patches.

We do enum and struct in a single patch because union tag enum and the
associated variants tie them together, and dealing with that to split
the patch doesn't seem worthwhile.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20181213123724.4866-18-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>