Audio: missing ability to disable microphone input from host?
Description of problem:
**It appears there is no way to disable the microphone / input to the audio backend device(s).**


There are at least two cases where this matters:
1. The host has no microphone input (e.g. only HDMI audio output with video).
2. The host has a microphone input, but the user doesn't want the guest VM to have access to the microphone/input.

I tried the option in.channels=0, as that seemed the most obvious way, though that doesn't work.

For -audio dsound, it appears that CLSID_DirectSoundCapture is unconditionally acquired.

There will also be later periodic warning/text outputs from QEMU "Could not create a backend for voice virtio.in", if you're running on a host system with no audio input device.

Adding a couple backend checks for channels > 0 may work well.  Not sure if it matters that audio front end device in the VM still thinks there is an audio input.