feat(vmm): add optional libvirt network filtering

[kvinwang]

Summary

• add an optional cvm.network_filter.mode = "libvirt" backend for bridge NICs; the default none mode preserves the existing unfiltered QEMU bridge-helper path
• keep QEMU launch and every attestation-sensitive QEMU argument under dstack-vmm; libvirt is used only for standalone nwfilter bindings
• add a small privileged dstack-vmm netd broker that creates deterministic TAPs, attaches them to the requested bridge, and creates/removes libvirt nwfilter bindings before/after QEMU
• support one shared host netd across multiple VMM instances through a stable instance namespace, while allowing dedicated sockets for development
• authenticate Unix-socket clients with SO_PEERCRED, bound protocol messages and fields, derive TAP names server-side, generate/escape binding XML internally, use fixed absolute host tools, and serialize operations across netd processes
• roll back all prepared NICs if setup or Supervisor deployment fails; remove bindings and TAPs after VM stop/removal
• let netd load only [netd], so it can use a small standalone root-owned configuration and does not initialize QEMU/VMM services
• support running both VMM and netd directly without systemd through --socket, --allow-uid, and --netd-socket

This implementation starts from master and does not depend on or reuse the implementation in #835/#836. Those PRs remain available only as historical reference.

Configuration

[cvm]
instance_id = "worker-a" # optional; derived from run_path when empty

[cvm.network_filter]
mode = "libvirt" # or "none" (default)
filter = "clean-traffic"
parameters = { IP = "192.0.2.10" }

[netd]
socket = "/run/dstack/netd.sock"
allowed_uids = [991, 992]
libvirt_uri = "qemu:///system"

When mode is none, dstack does not connect to netd and continues to emit:

-netdev bridge,id=netN,br=<bridge>

When mode is libvirt, netd finishes the binding before dstack submits QEMU, which receives:

-netdev tap,id=netN,ifname=<derived>,script=no,downscript=no,vhost=off

Verification

• cargo test -p dstack-vmm — 60 passed
• cargo clippy -p dstack-vmm --all-targets -- -D warnings
• cargo fmt --manifest-path vmm/Cargo.toml -- --check
• git diff --check origin/master...HEAD
• root integration test against the installed qemu:///system libvirt:
  • create temporary bridge
  • start the built dstack-vmm netd
  • prepare TAP with clean-traffic and static IP parameter
  • verify virsh nwfilter-binding-dumpxml
  • remove binding/TAP and verify both disappeared
• direct development-mode test with a minimal [netd] config and a non-root client authorized by UID

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