virsh: Set auto_close before raise Error in init of VirshSession

 If we failed to init VirshSession and catch a error. There will be a virsh process out of control, beacause default auto_close of VirshSession if False.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Merge pull request #366 from yangdongsheng/virsh_auto_close

virttest-virsh: Set auto_close before raise Error in init of VirshSession.