client.lv_utils: Add vg_list method.

Get volume groups with command "vgs --all", then analyse it to a dict:{vg_name: details_info,...}.

Sample:
{'vg_v2v': {'#SN': '0', 'Attr': 'wz--n-', '#LV': '0', 'VSize': '93.13g', 'VFree': '93.13g', '#PV': '1'}}