feat: complete KVM pre-fault stats

[Hygge-Gezelligheid]

Summary

Complete the paused-only KVM pre-fault path with synchronous, verifiable completion statistics.

• Continue KVM_PRE_FAULT_MEMORY until every submitted range reports zero remaining bytes.
• Return aggregate and per-vCPU-worker requested, completed, remaining, ioctl-count, and timing statistics.
• Avoid waiting for a response from a vCPU that could not be signaled.
• Expose the response through the Swagger API and parser models.

Validation

Passed on the AgentENV development host:

• cargo test -p vmm vstate::prefault --lib (9 passed)
• precise VMM integration coverage for state validation, multi-vCPU accounting, and hotpluggable-memory rejection (3 passed)
• cargo test -p firecracker api_server::parsed_request::tests::test_put_pre_fault_memory_route --lib

cargo fmt --check could not be used as a validation signal on that host because its installed toolchain reformats unrelated baseline files.