fw_cfg: Remove user_creatable flag

fw_cfg won't work with -device, as:
* fw_cfg_init1() won't get called for the device;
* The device won't appear at /machine/fw_cfg, and won't work with
  the -fw_cfg command-line option.

Remove the user_creatable flag from the device class.

Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Gabriel L. Somlo <somlo@cmu.edu>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Acked-by: NMarcel Apfelbaum <marcel@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-15-ehabkost@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

hw/nvram/fw_cfg.c

@@ -1101,11 +1101,6 @@ static void fw_cfg_io_class_init(ObjectClass *klass, void *data)
 
     dc->realize = fw_cfg_io_realize;
     dc->props = fw_cfg_io_properties;
-    /*
-     * FIXME: Set only because we are not sure yet if this device
-     * will be outside the q35 sysbus whitelist.
-     */
-    dc->user_creatable = true;
 }
 
 static const TypeInfo fw_cfg_io_info = {
@@ -1172,11 +1167,6 @@ static void fw_cfg_mem_class_init(ObjectClass *klass, void *data)
 
     dc->realize = fw_cfg_mem_realize;
     dc->props = fw_cfg_mem_properties;
-    /*
-     * FIXME: Set only because we are not sure yet if this device
-     * will be outside the q35 sysbus whitelist.
-     */
-    dc->user_creatable = true;
 }
 
 static const TypeInfo fw_cfg_mem_info = {