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>