OvmfPkg: AcpiPlatformDxe: when PCI is enabled, wait for Platform BDS's cue

OvmfPkg: AcpiPlatformDxe: when PCI is enabled, wait for Platform BDS's cue

This patch doesn't change the behavior of AcpiPlatformDxe when
PcdPciDisableBusEnumeration is TRUE -- that is, when the driver runs on
Xen (OvmfPkg and ArmVirtPkg both), or when the driver runs on QEMU as part
of ArmVirtPkg but no PCI host bridge was found by VirtFdtDxe. In these
cases the driver continues to install the ACPI tables immediately.

However, when PcdPciDisableBusEnumeration is FALSE (i.e., when the driver
runs on QEMU as part of OVMF, or as part of ArmVirtPkg and VirtFdtDxe
finds a PCI host bridge), we now delay the ACPI table download from QEMU.
We wait until the Platform BDS tells us that root bridges have been
connected, and PciIo instances are available.

The explanation is in the patch titled

  OvmfPkg: introduce gRootBridgesConnectedEventGroupGuid

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>