[3.15] main/xen: add mitigations for XSA-439
No patch supplied by upstream. Patch created with
git format-patch 0517763^..3a9a290 --output xen-stable-4.15-20230920.patch
No patch supplied by upstream. Patch created with
git format-patch 0517763^..3a9a290 --output xen-stable-4.15-20230920.patch