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.

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!)

[dorindabassey]

Please split the commit and improve the commit messages a bit, it's quite hard to follow.

Fixed, Thanks!

[mtjhrc]

The intermediary buffer doesn't seem useful - is there a reason why we can't just do this?

Suggested change

	let offset_usize = offset as usize;
	let end = offset_usize + data.len();
	let mut config_buf = vec![0u8; end];
	match frontend.get_config(
	0,
	end as u32,
	VhostUserConfigFlags::empty(),
	&mut config_buf,
	) {
	match frontend.get_config(
	offset as u32,
	data.len() as u32,
	VhostUserConfigFlags::empty(),
	data,
	) {

Sidenote: The protocol for VHOST_USER_GET_CONFIG is weird - I can't fathom why are we even sending the buffer (the last parameter of frontend.get_config) attached to the msg... But that concern is beyond libkrun project, nothing we can do about it directly here.

[dorindabassey]

right! there's no need for the intermediary buffer.