Add guest audio output backends

[serhatandic]

Summary

• add output-only virtio-sound for Apple Virtualization guests
• add Intel HDA output with SPICE audio for embedded UTM QEMU and Core Audio for VNC fallback
• play SPICE output natively through AVAudioEngine with retained, GLib-confined lifecycle and device-change recovery
• document output-only support and add configuration/lifecycle coverage

Validation

• make test
• make app-test (34 tests, 2 fixture-dependent skips)
• focused Apple audio configuration and SPICE connection lifecycle tests
• embedded UTM QEMU SPICE/HDA argument smoke via QMP

Caveats

• guest microphone/audio input forwarding is intentionally unsupported
• no per-VM mute control; output follows the host audio device and volume