Adapt to new (v2) VFIO migration interface

[w-henderson]

This PR will (when done):

• [x] Update to version of the spec as submitted to qemu-devel
• [x] Add new migration protocol based on VFIO migration v2, replacing the migration region with a more intuitive stream-like mechanism for transferring migration data (alongside other changes)
• [ ] Implement the new protocol in libvfio-user
• [ ] Update and add more tests for the new protocol
• [ ] Update the samples for the new protocol

Closes #654

[jlevon]

Hi William, thanks! Looks like a great start. Let's split things up from your branch into individual PRs - this will let us review each logical change on its own. Some context here: https://movementarian.org/blog/posts/github-prs/

As for the first change here, this looks good, but could you add some context to the commit message? In particular:

• mention where you're syncing from
• explain that while you're removing LM in the spec, the code will remain in the library until updated to v2 by later changes
• mention that we don't implement all of the other new changes in the spec update - but we are otherwise compatible
• as a useful side quest, would be good to file issues on any of those changes. For example we already have a PR for multi-write, but we definitely don't have issues filed for some of the new capabilities during negotiation

[swapnili]

@w-henderson Is this PR up for review or WIP I see that the build is failing? Are you going to split this into small logical PRs?

[w-henderson]

@swapnili This PR has since been split into the smaller #745 #746 #747 #754 which are at various different stages of development, I'll close this one since it's no longer relevant.