OpenVMM: give a port forward that cannot be bound at restore a stable error code #972

A clone of a template restores its NIC with its own network policy (#850). OpenVMM binds the host socket of each port forward of that policy as it resolves the NIC's endpoint, as the VM worker launches, before the restore-ready event. When it cannot bind one, for example because another socket holds the port, the worker's launch fails, and OpenVMM exits with a fatal error that carries no code:

fatal error: failed to launch vm worker

Caused by:
    0: ...
    1: failed to create socket for port forward (Tcp 127.0.0.1:PORT)
    2: <the operating system's error>

NVX attributes that failure to the host, with the evidence forward-bind, by finding the text failed to create socket for port forward ( among the causes of the error's chain. It pins the text with an excerpt of OpenVMM's source, nvx-rs/tests/vectors/openvmm/port_forward_socket_creation.rs, the ResolveConsommeError::SocketCreation variant of vm/devices/net/net_consomme/src/resolver.rs, which the ignored test nvx-rs/tests/openvmm_vectors.rs compares with the submodule. The text is not a contract, though: if OpenVMM rewords the error, the clone's failure falls back to a lost status of unknown outcome, and only the excerpt check of the next pin promotion notices.

Proposal

Give the error a stable bracketed code, as OpenVMM does for its time ABI errors, for example [E_PORT_FORWARD_BIND]. fatal_error_message in openvmm/openvmm_entry/src/microvm/verify.rs already leads the fatal error with the first bracketed E_ code of the chain, so the line would read fatal error: [E_PORT_FORWARD_BIND] failed to launch vm worker, and NVX would match the code at the start of the line, as it matches the time ABI codes of an incompatible host (INCOMPATIBLE_HOST_CODES in nvx-rs/src/host/template/clone/failure.rs).

Acceptance criteria

• OpenVMM on feature/microvm/dev reports a port forward whose host socket it cannot bind with a stable code that leads its fatal error, documented with its other stable codes, and tested.
• Once NVX pins that OpenVMM, the launch of a clone attributes the code to forward-bind, and the text match and the excerpt port_forward_socket_creation.rs are removed.