After (3), the __del__ problem is solved.
But another problem come with it.
We can use del vp to destroy a vp instance and close the related virshsession.
If there are two object vp_1 and vp_2 use the same virshsession.
del vp_1 will close the virshsession, and then vp_2 will not work correctly.

So, I introduce a COUNTERS in VirshSession class to counter the reference to virshsession.