Exit with an error (instead of simply logging a trace event)
whenever the same fw_cfg file name is added multiple times via
one of the fw_cfg_add_file[_callback]() host-side API calls.

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