throttle: Set always an average value when setting a maximum value

When testing the ranges of valid values, set_cfg_value() creates
sometimes invalid throttling configurations by setting bucket.max
while leaving bucket.avg uninitialized.

While this doesn't break the current tests, it will as soon as
we unify all functions that check the validity of the throttling
configuration.

This patch ensures that the value of bucket.avg is valid when setting
bucket.max.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
