Only start_vm="no" means we did not start vm, but if the vm is already running before we start test, this case will fail. That the reason we introduced the param named kill_vm_before_test, if we want to make sure the vm be killed before the test script, we should set kill_vm_before_test = "yes". But at the same time, you can see the commont in base.cfg, 
```
# If there is any conflict between 'start_vm' and 'kill_vm_before_test',
# the final desicion is made by start_vm.
```
So, if we want to use kill_vm_before_test = "yes", we should also set start_vm = "no". This design is about some compatibility. So, it looks wired, we should use the two params to make sure the vm is destroyed before tests script.