Balloons: use max CPU frequencies in shared idle CPUs

[askervin]

If workloads in a balloon are allowed to use idle CPUs through SharedIdleCpusInSame: <topology-level> option, the frequencies of the idle CPUs remains to be configured according to the idle CPU class. This may cause unexpected performance drop when a workload that usually runs in high frequency CPUs may be scheduled on a low frequency idle CPU.

This should be fixed so that the frequency of a CPU that any workload can run on should not drop due to being scheduled on a shared idle CPU. That is, shared idle CPUs should be configured to have the maximum frequency of CPU classes of any balloon that shares them.