OvmfPkg/XenSupport: use a correct PCI host bridge aperture for BAR64

In case BAR64 is placed below 4G choose the correct aperture.
This fixes a failed assertion down the code path.

Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>

OvmfPkg/Library/PciHostBridgeLib/XenSupport.c

@@ -137,7 +137,11 @@ PcatPciRootBridgeParseBars (
           Length = Length | LShiftU64 ((UINT64) UpperValue, 32);
           Length = (~Length) + 1;
 
-          MemAperture = MemAbove4G;
+          if (Base < BASE_4GB) {
+            MemAperture = Mem;
+          } else {
+            MemAperture = MemAbove4G;
+          }
         }
 
         Limit = Base + Length - 1;