Cancel running vcpus when exiting

[petrutlucian94]

We'll need to gracefully stop vcpus when the guest initiates a shutdown. One issue is that the WhpVirtualProcessor objects are moved to the vcpu runner loop and are no longer available afterwards.

A few ideas:

1. Have the vcpu loop use a WhpVirtualProcessor reference instead of owning it
   • it needs to become Send+Sync, which may not be safe/desired, considering that WhpVirtualProcessor embeds the emulator context
   • if the vcpu thread gets a WhpVirtualProcessor reference, rust requires it to have a static lifetime
2. Clone the WhpVirtualProcessor or libwhp;:VirtualProcessor objects
   • WhpVirtualProcessor owns a VirtualProcessor object, which implements Drop
   • WhpVirtualProcessor might become quite beefy, including objects that we use in order to emulate Kvm's API (e.g. pio/mmio events, etc)
3. Implement a "cancel_vcpu" method on the partition object
   • We'd need to handle errors raised when canceling an already destroyed vcpu
   • doesn't seem too clean as we're duplicating functionality