fw_cfg: remove support for guest-side data writes

fw_cfg: remove support for guest-side data writes

From this point forward, any guest-side writes to the fw_cfg
data register will be treated as no-ops. This patch also removes
the unused host-side API function fw_cfg_add_callback(), which
allowed the registration of a callback to be executed each time
the guest completed a full overwrite of a given fw_cfg data item.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>