Scheduler preemption hooks, #2

Changes since #2:
- better Kconfig
- documentation
- spinlock->mutex conversion from Shaohua

This patchset adds a general sceduler mechanism that allows tasks to
be notified when they are preempted and scheduled back in, and converts
kvm to use it.  The main benefits are that the kvm mmu can now perform
allocations, enabling features such as guest paging, and reducing preemption
latency to a bounded value for realtime workloads.

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