fw_cfg: Fix -boot reboot-timeout error checking

fw_cfg: Fix -boot reboot-timeout error checking

fw_cfg_reboot() gets option parameter "reboot-timeout" with
qemu_opt_get(), then converts it to an integer by hand. It neglects to
check that conversion for errors, and fails to reject negative values.
Positive values above the limit get reported and replaced by the limit.
This patch checks for conversion errors properly, and reject all values
outside 0...0xffff.

Signed-off-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <1542777026-2788-3-git-send-email-liq3ea@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>