Today only a few sysfs nodes under /sys/hypervisor/ are documented
for Xen in Documentation/ABI/testing/sysfs-hypervisor-pmu.
Add the remaining Xen sysfs nodes under /sys/hypervisor/ in a new
file Documentation/ABI/stable/sysfs-hypervisor-xen and add the Xen
specific sysfs docs to the MAINTAINERS file.