Xen: Obtain E820 memory map via XENMEM_memory_map
Xen: Obtain E820 memory map via XENMEM_memory_map

In order to support the custom Xen hypervisor [1] used by Qubes OS 4.0,
use the memory_op(XENMEM_memory_map) hypercall to obtain a memory map if
none is provided in (struct hvm_start_info). If neither an E820-style
nor a hvm_start_info-style memory map are provided then fail.

[1] Xen 4.8 with "most but not quite all" of PVHv2 backported to it:
https://github.com/QubesOS/qubes-vmm-xen/tree/xen-4.8

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