xen: fix isxenpmu()
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v4.14.276 by commit e9f72105cf4a16bb185093b60fc12dbc82fe15b5, it was introduced in version v4.3 by commit bf6dfb154d935725c9a2005033ca33017b9df439. For more details please see the references link.