virt-libvirt: Modify virsh dump test and some testcases

1.Instead of thread, use os.fork to create a new process,
  because we cannot get any exception in main process
  from another thread.
2.Add "try...finally" part at the end, because when test
  failed, config file won't be recoverd.
3.Add 3 test cases for paused VM test.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>