xen: implement hook to fetch and parse e820 memory map

e820 memory map is fetched using a hypercall under Xen PVH, so add a
hook to init_ops in oder to diverge from bare metal and implement a
Xen variant.

Approved by: gibbs
Sponsored by: Citrix Systems R&D

x86/include/init.h:

• Add a parse_memmap hook to init_ops, that will be called to fetch and parse the memory map.

amd64/amd64/machdep.c:

• Decouple the fetch and the parse of the memmap, so the parse function can be shared with Xen code.
• Move code around in order to implement the parse_memmap hook.

amd64/include/pc/bios.h:

• Declare bios_add_smap_entries (implemented in machdep.c).

x86/xen/pv.c:

• Implement fetching of e820 memmap when running as a PVH guest by using the XENMEM_memory_map hypercall.