env_process: preprocess_vm start_vm fix.

(1). Introduce a param named "kill_vm_before_test".
(2). Fix when restart_vm=yes.
(3). Fix when vm need to paused.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Add kill_vm_before_test=no in base.cfg

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>