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: Laszlo Ersek <lersek@redhat.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-15-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>