libvirt-virsh: update the using of VirshSession in VirshPersistent.

Introduce a class variable named COUNTERS in VirshPersistent. COUNTERS is a
dict whose key is a_id and value is counter. Two method counter_increase and
counter_decrease introduced in VirshPersistent at the same time. B/c VirshSession is
not auto closed, then we need to manage the VirshSession in VirshPersistent.

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