throttle: Test the valid range of config values

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: a57dd6274e1b6dc9c28769fec4c7ea543be5c5e3.1503580370.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
