[3.14] main/xen: security upgrade to 4.15.4

https://xenproject.org/downloads/xen-project-archives/xen-project-4-15-series/xen-project-4-15-4/

main/xen: add mitigations for XSA 326, 405, 406, 409-422

Currently this seems to be the only sure way to get all the bug and security fixes from the upstream stable branch. The command to reproduce the patch with commits between RELEASE-4.15.3 and the current tip of stable-4.15 is:

  git format-patch f6e26ce^..e818f4f \
      -output main/xen/xen-stable-4.15-20221101.patch

20221101 being the CommitDate of the current tip of the stable-4.15 branch.

https://xenbits.xen.org/xsa/advisory-326.html

https://xenbits.xen.org/xsa