Improve the way vCPU stop reason is implemented to reduce code duplication

[dblnz]

This is a follow up issue for the #327 PR.

1. Check the comments for more context.
   The way get_stop_reason method is implemented introduces code duplication between kvm and mshv, this can be improved.

2. it is worth investigating whether mshv has a way of checking which exception was intercepted.
   Link 1 and Link 2 are relevant to this.

3. Check whether gdb does not complain when an unknown vCPU exit reason is reported as SwBp