throttle: Don't allow burst limits to be lower than the normal limits

Setting FOO_max to a value that is lower than FOO does not make
sense, and it produces odd results depending on the value of
FOO_max_length. Although the user should not set that configuration
in the first place it's better to reject it explicitly.

https://bugzilla.redhat.com/show_bug.cgi?id=1355665

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reported-by: Gu Nini <ngu@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 663d5aca406060e31f80d8113f77b6feee63b919.1469693110.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
