libkrun: Add vhost-user sound device support

[dorindabassey]

Add support for vhost-user sound devices following
QEMU's vhost-user-snd implementation. This allows
libkrun VMs to use external vhost-user sound backends
(e.g., vhost-device-sound) for audio playback and capture.

The virtio-snd configuration space is read-only and
contains device capabilities (jacks, streams, chmaps, controls).
The implementation uses 4 queues as per the virtio-snd
specification: control, event, TX/playback, and RX/capture.

Note: The ALSA default device in the guest may not work without
additional configuration. Use explicit device specification
(e.g., 'aplay -D hw:0,0 ') or create /etc/asound.conf
in the guest to set the default device.

Depends on #527 and containers/libkrunfw#114

[mtjhrc]

Please split the commit and improve the commit messages a bit, it's quite hard to follow. It goes unnecessarily into internals of the virtio-sound spec ("The virtio-snd configuration space is read-only and contains device capabilities (jacks, streams, chmaps, controls).", but doesn't mention actual implementation details (I am wondering why do we need config_cache? - or at least mention existence of it).

Maybe split it into like 3 commits:

1. add config support for vhost user devices (this is mostly general, we need that for other vhost-user devices)
2. add the vhost-sound #define constants (and possibly other sound specific things)
3. add support in the chroot_vm example (you can mention how to get aplay to work in that commit message or in a comment somewhere that is great!)

[mtjhrc]

This is quite unfortunate, if we need to know the size of the config, I think it should be configured the same way as number and sizes of virtqueues are, preferably specified in the C API, with a constant for the known value.

But if we keep adding more parameters to krun_add_vhost_device(), maybe it makes sense for that function to accept a whole struct krun_vhost_config. (and provide entire configured values of the krun_vhost_config struct for each device type).