OvmfPkg: raise the PCIEXBAR base to 2816 MB on Q35

OvmfPkg: raise the PCIEXBAR base to 2816 MB on Q35

(This is a replacement for commit 75136b29541b, "OvmfPkg/PlatformPei:
reorder the 32-bit PCI window vs. the PCIEXBAR on q35", 2019-05-16).

Commit 7b8fe63561b4 ("OvmfPkg: PlatformPei: enable PCIEXBAR (aka MMCONFIG
/ ECAM) on Q35", 2016-03-10) claimed that,

  On Q35 machine types that QEMU intends to support in the long term, QEMU
  never lets the RAM below 4 GB exceed 2 GB.

Alas, this statement came from a misunderstanding that occurred while we
worked out the interface contract. In fact QEMU does allow the 32-bit RAM
extend up to 0xB000_0000 (exclusive), in case the RAM size falls in the
range (0x8000_0000, 0xB000_0000) (i.e., the RAM size is greater than
2048MB and smaller than 2816MB).

In turn, such a RAM size (justifiedly) triggers

  ASSERT (TopOfLowRam <= PciExBarBase);

in MemMapInitialization(), because we placed the 256MB PCIEXBAR at
0x8000_0000 (2GB) exactly, relying on the interface contract. (And, the
32-bit PCI window would follow the PCIEXBAR, covering the [0x9000_0000,
0xFC00_0000) range.)

In order to fix this, place the PCIEXBAR at 2816MB (0xB000_0000), and
start the 32-bit PCI window at 3 GB (0xC000_0000). This shrinks the 32-bit
PCI window to

  0xFC00_0000 - 0xC000_0000 = 0x3C00_0000 = 960 MB.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1859
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>