Add a virtio-net device driver #189

Summary

Provide a paravirtualized network interface using virtio-net so Rustalis can exchange packets in QEMU without depending on a physical NIC driver.

Scope

• Discover supported virtio-net PCI devices through the existing PCI/driver registry.
• Negotiate required virtio features and configure receive/transmit virtqueues.
• Validate descriptor chains and packet lengths before accessing buffers.
• Connect the driver to the network-device and packet-buffer abstractions.
• Define queue reset and device-error handling.

Acceptance criteria

• Feature negotiation rejects unsupported required features cleanly.
• Host tests cover descriptor validation and queue bookkeeping.
• QEMU integration instructions describe the required network device configuration.
• RX/TX failures are reported without corrupting queue state or leaking buffers.

Engineering constraints

• Keep the kernel Rust-first, no_std, and x86_64-compatible.
• Integrate with existing Rustalis subsystem interfaces; do not create a parallel implementation.
• Document ownership, initialization order, failure behavior, and any architecture-specific unsafe invariants.
• Add host-side tests for hardware-independent logic and QEMU coverage for hardware paths where practical.
• Do not claim verification that has not actually been run.