xen/x86: Use memblock_reserve for sensitive areas.

xen/x86: Use memblock_reserve for sensitive areas.

instead of a big memblock_reserve. This way we can be more
selective in freeing regions (and it also makes it easier
to understand where is what).

[v1: Move the auto_translate_physmap to proper line]
[v2: Per Stefano suggestion add more comments]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[upstream git commit 91addbf07abfdd109a9da4e02061e6ed3728b298]
Conflicts:

	arch/x86/xen/setup.c
[s/memblock_reserve/memblock_x86_reserve_range]