Document Xen XSAs-{142,148,149,150,151,152,153}

Security: CVE-2015-7311
Security: CVE-2015-7835
Security: CVE-2015-7969
Security: CVE-2015-7970
Security: CVE-2015-7971
Security: CVE-2015-7972
Security: https://vuxml.FreeBSD.org/freebsd/301b04d7-881c-11e5-ab94-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/3d9f6260-881d-11e5-ab94-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/83350009-881e-11e5-ab94-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/c0e76d33-8821-11e5-ab94-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/e3792855-881f-11e5-ab94-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/e4848ca4-8820-11e5-ab94-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/fc1f8795-881d-11e5-ab94-002590263bf5.html