throttle: Test burst limits lower than the normal limits

This checks that making FOO_max lower than FOO is not allowed.

We could also forbid having FOO_max == FOO, but that doesn't have
any odd side effects and it would require us to update several other
tests, so let's keep it simple.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 2f90f9ee58aa14b7bd985f67c5996b06e0ab6c19.1469693110.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
