xen: update port and apply security fixes
- Apply the following Xen security fixes (XSAs): 167, 168, 170.
- Update SeaBIOS version to 1.8.2, and apply build fix so it builds with ELF toolchain objcopy [0].
- Perform the backport of two functional changes to the Xen kernel in order to improve PVH Dom0 hardware support [1].
Security: CVE-2016-1570
Security: CVE-2016-1571
Security: CVE-2016-2271
Sponsored by: Citrix Systems R&D
Requested by: Gustau Pérez <gperez@entel.upc.edu> [1]
PR: 207170 [0]
Approved by: bapt
Differential revision: https://reviews.freebsd.org/D5420