vmm(4): treat vcpu lists as immutable, reducing complexity.

vmm(4): treat vcpu lists as immutable, reducing complexity.

Since vmm doesn't support hot-plug vcpus we can reduce complexity
by treating the vcpu list per vm as immutable after creation.

As a consequence, we can use the vm reference count to protect the
lifetime of the vcpus, removing the need for reference counting
individual vcpu objects. With an immutable list, we no longer need
a rwlock protecting it either.

Original diff from dlg@ that I reworked and tested.

ok dlg@, mlarkin@