fw_cfg: Make base type "fw_cfg" abstract

Missed when commit 5712db6a split off "fw_cfg_io" and "fw_cfg_mem".
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <1469777353-9383-1-git-send-email-armbru@redhat.com>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/nvram/fw_cfg.c

@@ -990,6 +990,7 @@ static void fw_cfg_class_init(ObjectClass *klass, void *data)
 static const TypeInfo fw_cfg_info = {
     .name          = TYPE_FW_CFG,
     .parent        = TYPE_SYS_BUS_DEVICE,
+    .abstract      = true,
     .instance_size = sizeof(FWCfgState),
     .class_init    = fw_cfg_class_init,
 };