virsh: Add ShellProcessTerminatedError in close_session.

When a VirshSession is already terminated when we call close_session(),
it will raise a ShellProcessTerminatedError. We should catch it and
notice that this session is already closed.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #1401 from yangdongsheng/VirshPersistent

virsh: Add ShellProcessTerminatedError in close_session.