Document several older QEMU vulnerabilities

Security: CVE-2015-7295
Security: CVE-2015-5278
Security: CVE-2015-5279
Security: CVE-2015-6855
Security: CVE-2015-6815
Security: CVE-2015-5239
Security: https://vuxml.FreeBSD.org/freebsd/42cbd1e8-b152-11e5-9728-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/6aa3322f-b150-11e5-9728-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/bbc97005-b14e-11e5-9728-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/10bf8eed-b14d-11e5-9728-002590263bf5.html
Security: https://vuxml.FreeBSD.org/freebsd/8a560bcf-b14b-11e5-9728-002590263bf5.html