Reset more vcpu state on snapshot::restore

[ludfjig]

MSRs will be added in another PR.

• Removed padding fields from fpu register to simplify comparisons. Not sure why I added them to begin with, we don't need C repr on these. It could possibly allow more efficient into() implementation for kvm/mshv due to single memcyp, but that seems like premature optimization to me
• Reset vcpu state on snapshot::restore
• Added bunch of tests
• MSRs are not included in this PR. Will add MSRs in future PR

Addresses #791 partially

------ After rebase ------

• Added field onto Snapshot which is SpecialRegisters. These are saved on snapshot() and restored when calling restore(). This is since we can no longer just reset them to default since the guest initializes them

[syntactically]

This looks pretty good to me! It's probably a good bit of extra overhead, but perhaps that's unavoidable.

One alternate option would be to require every guest to have some code somewhere that does some/all of this reset work inside the VM, validating that the correct code is in the correct place whenever we make a snapshot. We have talked about doing something similar for TLB flushes as well. That would let you batch most of this into a single hypercall, although the hypercall itself would be a bit more expensive. Do you have any sense of whether that would make sense to investigate?

[syntactically]

Sorry, missed one other comment.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 15 changed files in this pull request and generated 2 comments.