x86: Prefer multiboot-provided e820 over bios-provided e801 memory info.

Some UEFI systems do not provide e820 information. In this case we
should take the detailed memory map provided by a multiboot-capable
loader, rather than rely on very conservative values from the e801
bios call. Using the latter on any modern system really hardly makes
good sense.

[Excellent candidate for 4.1 backport]

Signed-off-by: Keir Fraser <keir@xen.org>
Tested-by: Jonathan Tripathy <jonnyt@abpni.co.uk>

xen/arch/x86/setup.c

@@ -690,17 +690,6 @@ void __init __start_xen(unsigned long mbi_p)
     {
         memmap_type = "Xen-e820";
     }
-    else if ( bootsym(lowmem_kb) )
-    {
-        memmap_type = "Xen-e801";
-        e820_raw[0].addr = 0;
-        e820_raw[0].size = bootsym(lowmem_kb) << 10;
-        e820_raw[0].type = E820_RAM;
-        e820_raw[1].addr = 0x100000;
-        e820_raw[1].size = bootsym(highmem_kb) << 10;
-        e820_raw[1].type = E820_RAM;
-        e820_raw_nr = 2;
-    }
     else if ( mbi->flags & MBI_MEMMAP )
     {
         memmap_type = "Multiboot-e820";
@@ -738,6 +727,17 @@ void __init __start_xen(unsigned long mbi_p)
             bytes += map->size + 4;
         }
     }
+    else if ( bootsym(lowmem_kb) )
+    {
+        memmap_type = "Xen-e801";
+        e820_raw[0].addr = 0;
+        e820_raw[0].size = bootsym(lowmem_kb) << 10;
+        e820_raw[0].type = E820_RAM;
+        e820_raw[1].addr = 0x100000;
+        e820_raw[1].size = bootsym(highmem_kb) << 10;
+        e820_raw[1].type = E820_RAM;
+        e820_raw_nr = 2;
+    }
     else if ( mbi->flags & MBI_MEMLIMITS )
     {
         memmap_type = "Multiboot-e801";