x86/xen: stop assuming kernel memory loading order in PVH

Do not assume that start_info will always be loaded at the highest
memory address, and instead check the position of all the loaded
elements in order to find the last loaded one, and thus a likely safe
place to use as early boot allocation memory space.

Reported by: markj, cperciva
Sponsored by: Citrix Systems R&D
Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D35628