Fixed error in test_hba_property end2end testcase

Details:

* The test_hba_property end2end testcase failed when trying to use the
  Partition.hbas property to invoke list(). That property is set to None
  on CPCs that have the storage management feature (i.e. z14 and later).

  It turned out that this error has always been there since the introduction
  of storage management feature support and only surfaced now when PR #1146
  added more exhaustive end2end testing also on existing functionality.

  Fixed by always initializing the Partition.hbas property even when on z14
  or later. Because the initialization is lazy, this does not affect the
  runtime, unless the hbas property is actually accessed.

Signed-off-by: Andreas Maier <maiera@de.ibm.com>

docs/changes.rst

@@ -87,6 +87,13 @@ Released: not yet
 
 **Cleanup:**
 
+* So far, the `Partition.hbas` property was set to `None` for CPCs that have the
+  "dpm-storage-management" feature enabled (i.e. starting with z14), because
+  HBAs are then represented as Virtual Storage Resource objects. For
+  consistency, this property was changed to provide an `HbaManager` object.
+  Since that property uses lazy initialization, there is no change at runtime
+  unless the property is actually accessed.
+
 **Known issues:**
 
 * See `list of open issues`_.

zhmcclient/_partition.py

@@ -259,17 +259,14 @@ def hbas(self):
         :class:`~zhmcclient.HbaManager`: Access to the :term:`HBAs <HBA>` in
         this Partition.
 
-        If the "dpm-storage-management" feature is enabled, this property is
-        `None`.
+        If the "dpm-storage-management" feature is enabled (i.e. starting with
+        z14), the CPC will not have any HBA objects anymore (they are now
+        Virtual Storage Resources), but this property still provides a manager
+        object for consistency.
         """
         # We do here some lazy loading.
         if not self._hbas:
-            try:
-                dpm_sm = self.feature_enabled('dpm-storage-management')
-            except ValueError:
-                dpm_sm = False
-            if not dpm_sm:
-                self._hbas = HbaManager(self)
+            self._hbas = HbaManager(self)
         return self._hbas
 
     @property