main/xen: add mitigations for XSA-407
https://xenbits.xen.org/xsa/advisory-407.html
About the xsa407-4.16-00.patch:
Upstream patches xsa407-4.16-*.patch are supposed to be applied after 744accad1b73223b3261e3e678e16e030d83b179, inferred from xsa407.meta, at the time of this writing the eleventh commit after the tip of the stable-4.16 branch. The command to reproduce the above intermediate patch is:
git format-patch 460b08d6^..744acca --stdout > xsa407-4.16-00.patch
Edited by omni