[lyakh]

Update May 27 2020: RPMsg now has a proper vhost-rpmsg implementation on the host instead of open-coding RPMsg messages. Also now rebased to the current sof-dev. vhost / rpmsg / virtio patches are here only for completeness, they are reviewed on respective mailing lists and should be pulled via respective maintainer trees.

Update May 15 2020: This version is using RPMsg for all of IPC, position updates and audio data. Of the original description below restrictions 2 and 4 have been fixed. 3 has been fixed largely. I would now send rpmsg and vhost patches to respective lists unless there are major objections. I didn't yet rebase my tree to the current sof-dev, so, this cannot be merged, this is for review or testing for now.

This is a replacement for the initial review-only #1269, this time it's "for real." This is still a WiP, here's a list of known limitations:

1. no zero-copy, using virtual queues for data and position updates
2. the only tested platform Up^2 has a problem with audio capture quality, so capture has only been tested conditionally
3. many string matching across the code, they have to be replaced with proper checks
4. rebooting the guest breaks virtualisation. Some resetting has to be implemented. Power off / on works.
   Of those at least (3) and (4) have to be fixed before merging. I'll add a link here in [DO NOT MERGE] Virtio wip 5 3 #1269 and close it.