Document Xen Security Advisories (XSAs 173, 175, 176, 178, 179, and 180).

XSAs 171, 172, 174, and 181 are not applicable to FreeBSD.

Discussed with: royger
Security: CVE-2014-3672
Security: CVE-2016-3710
Security: CVE-2016-3712
Security: CVE-2016-4963
Security: CVE-2016-4480
Security: CVE-2016-4962
Security: CVE-2016-3960
Security: https://vuxml.FreeBSD.org/freebsd/e800cd4b-4212-11e6-942d-bc5ff45d0f28.html
Security: https://vuxml.FreeBSD.org/freebsd/e6ce6f50-4212-11e6-942d-bc5ff45d0f28.html
Security: https://vuxml.FreeBSD.org/freebsd/e589ae90-4212-11e6-942d-bc5ff45d0f28.html
Security: https://vuxml.FreeBSD.org/freebsd/e43b210a-4212-11e6-942d-bc5ff45d0f28.html
Security: https://vuxml.FreeBSD.org/freebsd/e2fca11b-4212-11e6-942d-bc5ff45d0f28.html
Security: https://vuxml.FreeBSD.org/freebsd/d51ced72-4212-11e6-942d-bc5ff45d0f28.html