Only clear 'cancel_requested' flag when cancelling vcpu run.

[ludfjig]

Currently, it's possible for the cancel_requested flag to be cleared without the vcpu being cancelled. This can lead to a call to kill() being lost. Consider the following thread interleaving:

1. Vcpu finishes vcpu::run()
2. Other thread calls kill() which sets cancel_requested to true
3. Vcpu sets cancel_requested to false.
4. Other thread sends signal, which doesn't do anything because vcpu thread is in userspace.
5. Vcpu returns as normal

This PR fixes it by only clearing the flag when the vcpu is actually interrupted.