xen: fix XSAs

Add the following XSA patches: 159, 160, 162, 165, 166.

Security: CVE-2015-8339
Security: CVE-2015-8340
Security: CVE-2015-8341
Security: CVE-2015-7504
Security: CVE-2015-8555
PR: 205841
MFH: 2016Q1
Sponsored by: Citrix Systems R&D
Requested by: junovitch
Reviewed by: junovitch
Differential revision: https://reviews.freebsd.org/D4783