x86: Add support for PVH version 1 memmap

Version 0 of PVH booting uses a Xen hypercall to retrieve the system
memory map; in version 1 the memory map can be provided via the
start_info structure.

Using the memory map from the version 1 start_info structure allows
FreeBSD to use PVH booting on systems other than Xen, e.g. on the
Firecracker VM.

Reviewed by: royger
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D35800