libvirt-virsh: Remove SESSION_COUNTER from VirshPersistent.

As we use COUNTERS to manager VirshSession in VirshPersistent.
SESSION_COUNTER is no needed any more.
* remove SESSION_COUNTER from VirshPersistent.
* update virsh_unittest related with SESSION_COUNTER.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Merge pull request #751 from yangdongsheng/virsh_SESSION_COUNTER

libvirt-virsh: Remove SESSION_COUNTER from VirshPersistent.