storage/lvm: make the "hack" work with CallbackPool instances

CallbackPool instances are no ThinPool instances, but behave
identically, if their backend driver is a ThinPool instance.

qubes/storage/lvm.py

@@ -509,7 +509,7 @@ class ThinVolume(qubes.storage.Volume):
         # HACK: neat trick to speed up testing if you have same physical thin
         # pool assigned to two qubes-pools i.e: qubes_dom0 and test-lvm
         # pylint: disable=line-too-long
-        if isinstance(src_volume.pool, ThinPool) and \
+        if hasattr(src_volume.pool, 'thin_pool') and \
                 src_volume.pool.thin_pool == self.pool.thin_pool:  # NOQA
             yield from self._commit(src_volume.path[len('/dev/'):], keep=True)
         else:

qubes/tests/storage_lvm.py

@@ -792,7 +792,6 @@ class TC_00_ThinPool(ThinPoolBase):
 
         # also should be different than source volume (clone, not the same LV)
         self.assertNotEqual(uuid_after, source_uuid)
-        if VOLUME_CLASS == ThinVolume:
         self.assertEqual(self._get_lv_origin_uuid(volume.path), source_uuid)
 
         expected_revisions = {