With concurrency managed workqueues, use of dedicated workqueues
can be replaced by using system_wq. Drop periodic_controlvm_workqueue
by using system_wq.
Since there is only one work item periodic_controlvm_work and
different periodic_controlvm_works do not need to be ordered, increase
of concurrency level by switching to system_wq should not break anything.
cancel_delayed_work_sync() is used to ensure that work is not pending
or executing on any CPU.