[3.18] main/xen: add mitigations for XSA-439
No patch supplied by upstream. Patch created with
git format-patch 7d88979^..90c540c --output xen-stable-4.17-20230920.patch
No patch supplied by upstream. Patch created with
git format-patch 7d88979^..90c540c --output xen-stable-4.17-20230920.patch