Document Xen Security Advisories (XSAs 159, 160, 162, 165, 166)

PR: 205841
Security: CVE-2015-8555
Security: CVE-2015-8341
Security: CVE-2015-8339
Security: CVE-2015-8340
Security: https://vuxml.FreeBSD.org/freebsd/6aa2d135-b40e-11e5-9728-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/e839ca04-b40d-11e5-9728-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/5d1d4473-b40d-11e5-9728-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/bcad3faa-b40c-11e5-9728-002590263bf5.html