fix: resolve TODO to use stateless structs for public VMM code

[aaron-ang]

use VmmActions with (entrypoint, args) tuples and spec struct suffix.

Changes

• Renamed public VMM inputs from *Config/cfg to *Spec/spec. Using “spec” suffix makes the contract explicit.
• VMM entry points take static specs as arguments, while internal state is derived privately.

Reason

Address a TODO regarding public-facing VMM structs. See #3273.

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• This functionality cannot be added in rust-vmm.