In XS 7.5, the xapi-core RPM owned the following files:
But it did not own either /etc/xenserver no /etc/xenserver/features.d (it should have owned both since no other package provides them), so when I updated XS 7.5 to 7.6 with the update ISO and xapi-core was updated to a version that doesn't have those corosync and network_sriov files anymore, /etc/xenserver/features.d was left behind, empty.
Similarly, the update of xapi-storage left the owned-by-nobody /usr/lib/python2.7/site-packages/xapi/storage/api/v4/ directory behind.
Note: there are probably many other packages that create directories that neither they nor any other package owns from a package management point of view. They can all lead to that kind of situation in future updates. It's usually harmless but knows what can happen in the future if some file or directory is left somewhere where no one expects them to be? I won't report all those that I found, unless you want me to.