at_dt_iscsi_disk: change the way of checking return for snapshot_info/snapshot_list

Duo to the return dict object of virsh.snapshot_info/virsh.snapshot_list
has no attribute 'exit_status', libvirt.check_exit_status can't be used to
check the return of virsh.snapshot_info/virsh.snapshot_list.
Merge pull request #357 from PandaWei/snapshot

at_dt_iscsi_disk: change the way of checking return for snapshot_info/sn...