fw_cfg: add write callback

fw_cfg: add write callback

Reintroduce the write callback that was removed when write support was
removed in commit 023e3148567ac898c7258138f8e86c3c2bb40d07.

Contrary to the previous callback implementation, the write_cb
callback is called whenever a write happened, so handlers must be
ready to handle partial write as necessary.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>