virtio: add vhost-user sound device support
Add a vhost-user sound device to cloud-hypervisor, enabling audio
passthrough via an external vhost-user sound backend daemon.
The implementation mirrors the existing vhost-user GPU device. The
sound device uses 4 virtqueues (control, event, tx, rx) per the virtio
sound specification, and sets SCHED_FIFO priority 88 on its epoll
thread for low-latency audio processing.
Changes span the full stack:
- vm-virtio: Sound = 25 device type registration
- virtio-devices: Sound device implementation, seccomp filters,
ActivateError variant
- vmm: SoundConfig, CLI --sound argument, OpenAPI spec, device manager
wiring, seccomp filters updated for sched_setscheduler
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>