Xen: retrieve mem_size via a hypercall and the E820 map
Xen: retrieve mem_size via a hypercall and the E820 map

This unifies the three approaches to memory map:
- multiboot
- start_info
- E820

Since the hypercall and E820 map works in both settings, cut the code complexity
and always use the XENMEM_memory_map.

See https://github.com/QubesOS/qubes-issues/issues/6162 and
https://github.com/mirage/qubes-mirage-firewall/issues/120

Solo5/solo5,Solo5/solo5