x86/xen: fix DomU boot problem

Construct Xen guest e820 map with a hole between 640K-1M.

It's pure luck that Xen kernels have gotten away with it in the past.

The patch below seems like the right thing to do. It certainly boots in
a domU without the DMI problem (without any of the other related patches
such as Alexander's).

Signed-off-by: Ian Campbell <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Jeremy Fitzhardinge <[e-mail]>
Tested-by: Mark McLoughlin <[e-mail]>
Acked-by: Mark McLoughlin <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Acked-by: Thomas Gleixner <[e-mail]>

arch/x86/xen/setup.c

···
 	unsigned long max_pfn = xen_start_info->nr_pages;
 
 	e820.nr_map = 0;
-	add_memory_region(0, PFN_PHYS(max_pfn), E820_RAM);
+	add_memory_region(0, LOWMEMSIZE(), E820_RAM);
+	add_memory_region(HIGH_MEMORY, PFN_PHYS(max_pfn)-HIGH_MEMORY, E820_RAM);
 
 	return "Xen";
 }