fix(vmm): keep Host API on the private vsock listener

[kvinwang]

Problem

VMM could expose the Host API on a general/public vsock listener after listener setup was refactored. That crossed the intended trust boundary: guest-facing peers could reach host-management RPCs.

Root cause and fix

Keep Host API routing attached only to the private vsock listener and leave the public listener with its intended guest surface.

Implementation

The branch records the following focused implementation work:

• fix(vmm): keep Host API on private vsock listener

Changed paths:

• dstack/vmm/src/main.rs

Scope

This PR addresses one logical vmm finding. It intentionally excludes the acceptance-test infrastructure from #841 and unrelated product fixes from #840.

Dependency and merge order

This PR is based directly on master and does not require another split product PR to merge first.

Verification

• git diff --check origin/master..origin/codex/fix-vmm-private-host-api: passed.
• The declared base was verified as an ancestor of the PR head.
• Focused compile/check verification was run for the changed component where applicable; non-Rust packaging or configuration changes were reviewed against their exact branch delta.

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