Enable Firecracker logging and metrics (named pipes, FlushMetrics) and set up guest-host communication over vsock

Documented steps

1. Logger: create the backing resource first (`mkfifo logs.fifo` or `touch logs.file`), then configure ONCE: `curl --unix-socket /tmp/firecracker.socket -X PUT http://localhost/logger -d '{"log_path": "logs.fifo", "level": "Warning", "show_level": false, "show_log_origin": false}'` — or at startup via `--log-path logs.fifo --level Error --show-level --show-log-origin`.
2. Metrics: `mkfifo metrics.fifo`, then `curl --unix-socket /tmp/firecracker.socket -X PUT http://localhost/metrics -d '{"metrics_path": "metrics.fifo"}'` or CLI `--metrics-path metrics.fifo`; the PUT body also accepts "emit_id": true (tags lines with the --id) and "properties": {...} custom key/values.
3. Metrics flush automatically every 60 seconds; flush on demand with `curl --unix-socket /tmp/firecracker.socket -X PUT http://localhost/actions -d '{"action_type": "FlushMetrics"}'`.
4. Read fifos in a loop (`read line <$logs`); plain files can just be cat'ed.
5. Vsock (pre-boot): `curl --unix-socket /tmp/firecracker.socket -X PUT http://localhost/vsock -d '{"guest_cid": 3, "uds_path": "./v.sock"}'`.
6. Host-initiated connections: connect to the uds_path AF_UNIX socket and send "CONNECT <port_num>\n"; if a guest listener exists on that AF_VSOCK port, Firecracker replies "OK <assigned_hostside_port>\n" and forwards; otherwise it terminates the host connection.
7. Guest-initiated connections: the guest connects to HOST_CID (2) and a port; Firecracker forwards to an AF_UNIX socket that must ALREADY be listening at `<uds_path>_<port>` (e.g. ./v.sock_52); if nothing listens, the guest gets VIRTIO_VSOCK_OP_RST.
8. Verify support: host kernel CONFIG_VHOST_VSOCK=m, guest kernel CONFIG_VIRTIO_VSOCKETS=y, and `ls /dev/vsock` succeeds in the guest.
9. Official docs: https://github.com/firecracker-microvm/firecracker/blob/main/docs/logger.md, https://github.com/firecracker-microvm/firecracker/blob/main/docs/metrics.md, https://github.com/firecracker-microvm/firecracker/blob/main/docs/api_requests/actions.md, https://github.com/firecracker-microvm/firecracker/blob/main/docs/vsock.md