Remove hypervisor_handler thread

[ludfjig]

• Removes the per-vm hypervisor_handler thread in favor of running on the callers thread.
• Removes all timeout-based cancellation in favor of an explicit kill() api. Currently, one can only cancel guest function calls, not the first vm initialization, but this should be fine as long as you trust your guest binary. You can only interrupt a guest while it's in a blocking call to vcpufd.run(). Host function calls can still not be interrupted (Make it possible to kill guest execution when running a host function. #192)

These changes should improve performance and throughput. It should also avoid the incredible performance drop off we observed under load when the hypervisor handler thread required joining, after cancelling guest execution. 

Added API changes:

impl MultiUseSandbox{
    /// Get a handle to the interrupt handler for this sandbox,
    /// capable of interrupting guest execution.
    pub fn interrupt_handle(&self) -> Arc<dyn InterruptHandle> {
         ...  
    }
    ...
}

/// A trait for handling interrupts to a sandbox's vcpu
pub trait InterruptHandle: Send + Sync {
    /// Interrupt the corresponding sandbox's vcpu if it's running.
    ///
    /// - If this is called while the vcpu is running, then it will interrupt the vcpu and return `true`.
    /// - If this is called while the vcpu is not running, then it will do nothing and return `false`.
    ///
    /// # Note
    /// This function will block for the duration of the time it takes for the vcpu thread to be interrupted.
    fn kill(&self) -> bool;

    /// Returns true iff the corresponding sandbox has been dropped
    fn dropped(&self) -> bool;
}

Removed API changed:

• All timeout based configuration

closes #471

Note: On KVM, moving the vcpufd (sandbox) to a new thread will incur a performance overhead the first time the vcpu in ran on the new thread, as per kvm kernel docs

[simongdavies]

Left some comments/questions , not quite finished reviewing , I will finish it in the morning

[dblnz]

Awesome work! 💯
I tested it locally. 😺

[simongdavies]

LGTM , nice work @ludfjig

[danbugs]

Looking good. Thanks for the fix on the SIGRTMIN offset stuff!

To finish off this PR, I think we just have to update some docs. Doing a quick search for "hypervisor handler" in the project shows references in shared_mem.rs, guest.rs, how-to-debug-a-hyperlight-guest.md, and the copilot-instructions.md.

Also, I think we should update the signal-handlers-development-notes.md doc to be appropriate to what we have rn.

[danbugs]

LGTM!!