OvmfPkg: AcpiPlatformDxe: enable PCI IO and MMIO while fetching QEMU tables

OvmfPkg: AcpiPlatformDxe: enable PCI IO and MMIO while fetching QEMU tables

Now that the previous patches ensure that we can access all PCI devices in
AcpiPlatformDxe, we can enable IO and MMIO decoding for all of them while
we contact QEMU for the ACPI tables. See more details in the patch titled:

  OvmfPkg: introduce gRootBridgesConnectedEventGroupGuid

In particular, this patch will prevent the bug when the 64-bit MMIO
aperture is completely missing from QEMU's _CRS, and consequently Linux
rejects 64-bit BARs with the error message

  pci 0000:00:03.0: can't claim BAR 4 [mem 0x800000000-0x8007fffff 64bit
                    pref]: no compatible bridge window

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>