fix virtio console reactivation after reset

[jonaswre]

Problem

The virtio-console backend treats activation as one-shot and leaves ports marked active after shutdown. After a valid virtio reset, a second guest activation is not handled and the port worker threads are not restarted. This breaks Linux hibernation/resume and any other device reset lifecycle.

Fix

• Keep the console activation event registered across reset cycles.
• Transition a shut-down port to Inactive before joining its workers so the next PORT_OPEN restarts them.
• Add regression coverage for both reset invariants.

Validation

• cargo test -p msb_krun_devices --lib (79 passed)
• Live Microsandbox Linux hibernation restore using this commit: restore 2.18s; first exec 0.03s; runtime virtio-fs share and heartbeat restored.
• cargo clippy -p msb_krun_devices --lib --tests -- -D warnings was attempted but is blocked by 14 pre-existing warnings in unrelated legacy/descriptor tests under Rust 1.97.0.