avocado/utils/lv_utils.py: use the CmdException correct attribute name

It doesn't have a 'result_obj' attribute, but it does have a `result`
attribute which should be an instance of
`avocado.utils.process.CmdResult`.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/utils/lv_utils.py

@@ -444,7 +444,7 @@ def lv_take_snapshot(vg_name, lv_name,
         process.run(cmd, sudo=True)
     except process.CmdError as ex:
         if ('Logical volume "%s" already exists in volume group "%s"'
-            % (lv_snapshot_name, vg_name) in ex.result_obj.stderr and
+            % (lv_snapshot_name, vg_name) in ex.result.stderr and
             re.search(re.escape(lv_snapshot_name + " [active]"),
                       process.run("lvdisplay", sudo=True).stdout)):
             # the above conditions detect if merge of snapshot was postponed