cpu: fix the privsep issue when offlining the cpu

In Icb913ed9be8d508de35e755a9c650ba25e45aca2 we forgot to add a privsep
decorator for the set_offline() method.

Change-Id: I769d35907ab9466fe65b942295fd7567a757087a
Closes-Bug: #2022955
(cherry picked from commit 3fab437)

Author: sbauza

nova/virt/libvirt/cpu/core.py

@@ -60,6 +60,7 @@ def set_online(core: int) -> bool:
     return get_online(core)
 
 
+@nova.privsep.sys_admin_pctxt.entrypoint
 def set_offline(core: int) -> bool:
     filesystem.write_sys(os.path.join(gen_cpu_path(core), 'online'), data='0')
     return not get_online(core)

releasenotes/notes/bug-2022955-eac8b0c8e42e3696.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    The `CPU power management`_ feature has been fixed to use privsep to avoid
+    a FileNotFound error when offlining CPUs.
+
+    .. _CPU power management: https://docs.openstack.org/nova/latest/admin/cpu-topologies.html#configuring-cpu-power-management-for-dedicated-cores