Fix/kvm clock and iovec rw

[kilroyjones]

Changes

The first fix prevents guest soft lockup panics on resume by implementing KVM_KVMCLOCK_CTRL, which signals the guest kernel to reset its watchdog clock after a pause. This manual implementation also includes a new metric to track kvmclock_ctrl failures.

The second change modifies IoVecBuffer and IoVecBufferMut to support partial I/O operations by returning the total bytes processed rather than failing completely if an error occurs mid-stream. This ensures that any data successfully read or written before an interruption is acknowledged and returned to the caller.
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Reason

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PR Checklist

• If a specific issue led to this PR, this PR closes the issue.
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this
  PR.
• API changes follow the Runbook for Firecracker API changes.
• User-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.
• New TODOs link to an issue.
• Commits meet
  contribution quality standards.

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• This functionality cannot be added in rust-vmm.

[reijne]

I cannot comment on the file but in src/vmm/src/vstate/vcpu/mod.rs there exists a TODO comment to use the functionality that I believe we have introduced now as a function:

fn running(&mut self) -> StateMachine<Self> {
...
// Break this emulation loop on any transition request/external event.
        match self.event_receiver.try_recv() {
            // Running ---- Pause ----> Paused
            Ok(VcpuEvent::Pause) => {
                // Nothing special to do.
                self.response_sender
                    .send(VcpuResponse::Paused)
                    .expect("vcpu channel unexpectedly closed");

                // TODO: we should call `KVM_KVMCLOCK_CTRL` here to make sure
                // TODO continued: the guest soft lockup watchdog does not panic on Resume.

                // Move to 'paused' state.
                state = StateMachine::next(Self::paused);
            }