Implement Cpc.get_wwpns() for z14 and higher

[andy-maier]

In the zhmcclient, we are providing the "Export WWPN List" operation as Cpc.get_wwpns().

The WS-API book states that with the "dpm-storage-management" feature enabled (i.e. z14 and later), the "Export WWPN List" operation fails with HTTP 409 (conflict).

We incorrectly state the supported environments for that method as SE>=2.13, which should actually be SE==2.13 because it is not supported at the WS-API for z14 and later. I think we can actually implement the iterations I described above on z14 and higher, so that the method works on all SE/CPC versions. The approach for getting the WWPNs on z14 and later is to iterate through the storage groups attached to the partition, then through its VSRs, to get the wwpn property of the VSR.