libvirt-virsh: fix __call__ in VirshClosure to avoid update dargs in VirshPersistent.

Previously, in VirshClosure.__call__(), weakref to VirshPersistent will update the VP.
It will change or modify the slots in VirshPersistent instance, then make the dargs
to next __call__().

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