Log Message:
Update xenkernel415, xentools415 and xenstoretools to 4.15.4
Main changes from 4.15.3:
includes all security fixes up to and including XSA-422
other changes are minor improvements and fixes.
Complete changelog can be found at:
https://xenproject.org/downloads/xen-project-archives/xen-project-4-15-series/xen-project-4-15-4/