Fix soundness issues via `KvmRunWrapper::as_mut_ref()`

[00xc]

Summary of the PR

As detailed in #248, and since KvmRunWrapper implements Send and Sync, as_mut_ref() can cause undefined behavior, as two threads can acquire a mutable reference to the kvm_run struct via an immutable reference to KvmRunWrapper.

Fix this by making KvmRunWrapper::as_mut_ref() take &mut self, which also gets rid of a clippy warning suppression, and update the callers. This results in potentially breaking changes in the public interface, as several VcpuFd methods now take &mut self as well.

Fixes: #248

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] 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.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

[00xc]

Ping for the remaining reviewers.

[JonathanWoollett-Light]

This is a breaking change so it would be good to check if anyone has any other concerns, otherwise LGTM. I've messaged in the rust-vmm public slack, if there are no concerns after some time I will approve.

[00xc]

Perhaps there should be a security advisory for this. Producing undefined behavior is pretty simple:

let vcpu = ...;

// Suppose this is a VcpuExit that contains an immutable
// reference to kvm_run, e.g. IoOut, MmioWrite
let exit = vcpu.run().unwrap();

// Suppose this is a VcpuExit of the same type as above.
// This modifies kvm_run again.
vcpu.run().unwrap();

// The value `exit` immutably references has been modified,
// yet the compiler does not complain because `run()` takes
// `&self`. Using `exit` here is UB.
println!("{:?}", exit);

Another way to think about it is that mutable accesses must invalidate all previous immutable references.