doc,xen: document hypervisor sysfs nodes for xen

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.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>