Use the proper exception in cgroup test

The process library uses it's own "process.CmdError" exception.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cgroup.py

@@ -1665,7 +1665,7 @@ def run(test, params, env):
                 try:
                     out = process.system_output("cat /proc/%s/task/*/stat" %
                                                 pid)
-                except exceptions.CmdError:
+                except process.CmdError:
                     out = None
                 else:
                     break