qapi: break long lines at 'data' member

Let's break the line before 'data'. While at it, improve a bit
indentation/spacing. (I removed some alignment which are not helping
much readability and become quickly inconsistent)

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181208111606.8505-26-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>