[RFC] Add support for `KVM_GET_XSAVE2` ioctls

[roypat]

Since Linux 5.17, the kvm_xsave struct has a flexible array member (FAM) at the end, which can be retrieved using the KVM_GET_XSAVE2 ioctl [1]. What makes this FAM special is that the length is not stored in the header, but has to be retrieved via KVM_CHECK_CAPABILITY(KVM_CAP_XSAVE2), which returns the total size of the kvm_xsave struct (e.g. the traditional 4096 byte header + the size of the FAM). Thus, to support KVM_GET_XSAVE2, we first need to check the capability on the VM fd, construct a FamStructWrapper of the correct size, and then call KVM_GET_XSAVE2.

I'm marking this as "RFC" because I'm not quite sure how to best deal the combination of "check capability on VM FD" and "do ioctl on vcpu FD". In this patch, I simply add a VmFd parameter to Vcpu::get_xsave2, but that's kinda ugly. Alternatively, we could store a reference to the VM file descriptor inside each Vcpu structure, but that'd be a larger change. What do people think about this?

(also, this PR needs https://github.com/rust-vmm/kvm-bindings/pull/104, because Rust apparently only type checks type aliases at use-sites instead of definition-site, meaning I did not notice that I forgot to implement Default for kvm_xsave2 when working on the new kvm-bindings release. That's why the CI is failing here for now)

Requirements

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• [ ] All commits in this PR have Signed-Off-By trailers (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [ ] All added/changed functionality has a corresponding unit/integration test.
• [ ] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [ ] Any newly added unsafe code is properly documented.

[roypat]

cc @likebreath since iirc you were looking into a post-5.17 solution for kvm_xsave, too