qapi2texi: Use category "Object" for all object types

At the protocol level, the distinction between struct, flat union and
simple union is meaningless, they are all JSON objects. Document them
that way.

Example change (qemu-qmp-ref.txt):

- -- Simple Union: InputEvent
+ -- Object: InputEvent

Input event union.

This also fixes the completely broken headings for flat and simple
unions in qemu-qmp-ref.7 and qemu-ga-ref.7, by sidestepping a bug in
texi2pod.pl. For instance, it mistranslates "@deftp {Simple Union}
InputEvent" to "B<Union> (Simple)", but translates "@deftp Object
InputEvent" to "B<SocketAddress> (Object)".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-30-git-send-email-armbru@redhat.com>