VirtIO sound TODOs

[alexandermbrown]

Future TODOs following this PR #37

• Merge examples/virtio-sndwith examples/virtio.
  Tracking issue: Merge virtIO and virtIO sound examples together #60. This will involve multiplexing the shared bus on the OdroidC4.

• Virtualiser ownership bug:
  Currently if you play audio in a client VM then immediately stop the stream (CTRL+C) the virtualiser it sometimes errors saying "invalid owner id -1". This is probably something to do with it getting released before all frames have been responded to. Note you need to play a long sound file (e.g., cantina.c from old commit) to get this to appear.

• WFI Interrupt:
  I have implemented a prototype implementation which handles aarch64's "wait for interrupt" instruction to suspend the VM until a notification is recieved (on https://github.com/au-ts/libvmm/tree/virtio_snd_wfi). Currently the VMM gets a WFI but never gets a notification to wake the VM. This needs to be investigated.

• Benchmarking:
  I implemented a benchmark which measures CPU usage of a native client client, virtualiser and driver VM (on https://github.com/au-ts/libvmm/tree/virtio_snd_bench). This needs to be redone with WFI interrupt handling.
  

• Passthrough IRQ 5:
  snd_driver_vmm.c contains passthrough IRQ 5. We are not sure what this is but Linux doesn't boot without it.

• Use libuio:
  The userlevel driver doesn't use our UIO library at the moment. It may need some modifications to integrate.

• Additional virtIO sound features:
  We currently don't support virtIO sound features such as the event queue, jacks or channel maps. This should be investigated.

• Generic queues (ivanv):
  I don't like that we have multiple generic queue implementations, would be good to avoid if possible.

• Userlevel program compilation:
  We currently use zig cc to build userlevel programs as I have been unable to get aarch64-linux-gnu-gcc or clang to work on macOS with Nix. Ideally zig wouldn't be a requirement.

• CI simulation
  It would be good if the virtio-snd example was run and tested in the CI. A possible solution is described here ci: add virtio-snd building #87 (comment)