ArmVirtualizationPkg/PciHostBridgeDxe: abort if there's no PCI host bridge
    
    If VirtFdtDxe found no PCI host in the DTB, then the config space base
    address will be left at zero -- the default is set in the DSC --, and we
    should exit PciHostBridgeDxe immediately.
    
    This causes gEfiPciRootBridgeIoProtocolGuid not to be installed, which in
    turn prevents MdeModulePkg/Bus/Pci/PciBusDxe from binding (see
    PciBusDriverBindingSupported()).
    
    That matches our intent, but then we also need to notify
    OvmfPkg/AcpiPlatformDxe that it must not wait for PCI enumeration
    completion.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>