KVM: document lock orders

1. Acquisition Orders

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(to be written)

The acquisition orders for mutexes are as follows:

- kvm->lock is taken outside vcpu->mutex

- kvm->lock is taken outside kvm->slots_lock and kvm->irq_lock

- kvm->slots_lock is taken outside kvm->irq_lock, though acquiring

  them together is quite rare.

For spinlocks, kvm_lock is taken outside kvm->mmu_lock.  Everything

else is a leaf: no other lock is taken inside the critical sections.

2: Exception

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