virt: Fix a bug in preprocess_vm func

If VM is running before test, and param.get('start_vm') == 'yes',
the variable 'start_vm' will be False, at this situation
params.get("kill_vm_before_test") will work, if it's 'yes',
the VM will be shutdown, and make the test fail.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Pls read the comment in base.cfg about kill_vm_before_test. 
```
# If there is any conflict between 'start_vm' and 'kill_vm_before_test',
# the final desicion is made by start_vm.
```
If you want to kill vm before test, you should set (start_vm=no and kill_vm_before_test=yes). 
It means kill_vm_before_test only works in start_vm is no.

What do you want with your patch here?