fw_cfg: Allow reboot-timeout=-1 again

fw_cfg: Allow reboot-timeout=-1 again

Commit ee5d0f89de3e53cdb0dc added range checking on reboot-timeout
to only allow the range 0..65535; however both qemu and libvirt document
the special value -1  to mean don't reboot.
Allow it again.

Fixes: ee5d0f89de3e53cdb0dc ("fw_cfg: Fix -boot reboot-timeout error checking")
RH bz: https://bugzilla.redhat.com/show_bug.cgi?id=1765443
Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Message-Id: <20191025165706[email protected]>
Suggested-by: Laszlo Ersek <[email protected]>
Message-Id: <37ac197c-f20e-dd05-ff6a-13a2171c7148@redhat.com>
[PMD: Applied Laszlo's suggestions]
Reviewed-by: Laszlo Ersek <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>