KVM machine.Get(): Use up vCPU pool before scanning for available ones.
KVM machine.Get(): Use up vCPU pool before scanning for available ones.

Previously, we created vCPUs on demand, and so taking an unused vCPU from the
pool was expensive. Now, we pre-create all vCPUs at the start, so it makes
sense to use them all to reduce contention.

PiperOrigin-RevId: 448382485

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