[3.17] main/xen: add mitigations for XSA-439
No patch supplied by upstream. Patch created with
git format-patch d720c23^..c450a4b --output xen-stable-4.16-20230920.patch
No patch supplied by upstream. Patch created with
git format-patch d720c23^..c450a4b --output xen-stable-4.16-20230920.patch