qapi2texi: Generate reference to base type members

The generated documentation doesn't mention object type members
inherited from a base type.  Fix that.

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

  -- Struct: VncServerInfo

      The network connection information for server

      Members:
      'auth' (optional)
	   authentication method used for the plain (non-websocket) VNC
	   server
+     The members of 'VncBasicInfo'

      Since: 2.1

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-27-git-send-email-armbru@redhat.com>
