xen: update to 4.5.1
Update xen-kernel to 4.5.1 and add patches to allow live migration, save and
restore. Remove qemu-traditional patches (FreeBSD doesn't support
qemu-traditional) and add XSA-142.
Approved by: bapt
Differential revision: https://reviews.freebsd.org/D3854
Sponsored by: Citrix Systems R&D