OvmfPkg/PciHostBridgeLibScan: remove QEMU (fw_cfg) support
    
    The "OvmfPkg/Library/PciHostBridgeLibScan/PciHostBridgeLibScan.inf"
    instance is used in the following platforms in edk2:
    
      OvmfPkg/Bhyve/BhyveX64.dsc
      OvmfPkg/OvmfXen.dsc
    
    Both platforms define "PcdPciDisableBusEnumeration" with Fixed-at-Build
    access method, and TRUE value. Remove the PCD from the
    PciHostBridgeLibScan instance, and everything else that is useful only
    when the PCD is FALSE.
    
    In practice, this removes the PciHostBridgeUtilityGetRootBridges()
    function call, which is based on fw-cfg; see
    "OvmfPkg/Library/PciHostBridgeUtilityLib/PciHostBridgeUtilityLib.c".
    
    (Note that the dependency on PciHostBridgeUtilityLib remains in place,
    given that the PciHostBridgeLibScan instance continues using lower-level
    functions from the library that do not depend on fw-cfg.)
    
    Cc: Anthony Perard <anthony.perard@citrix.com>
    Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Julien Grall <julien@xen.org>
    Cc: Peter Grehan <grehan@freebsd.org>
    Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
    Cc: Rebecca Cran <rebecca@bsdio.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Message-Id: <20210526201446.12554-34-lersek@redhat.com>
    Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
    Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>