fw_cfg: Remove user_creatable flag

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 <[email protected]>
Cc: "Michael S. Tsirkin" <[email protected]>
Cc: Laszlo Ersek <[email protected]>
Cc: Gabriel L. Somlo <[email protected]>
Reviewed-by: Laszlo Ersek <[email protected]>
Acked-by: Marcel Apfelbaum <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>
Message-Id: <20170503203604[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>