OvmfPkg/PciHostBridgeLib: remove Bhyve and Xen support
    
    The "OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.inf" instance is
    used by the following platforms in edk2:
    
      OvmfPkg/AmdSev/AmdSevX64.dsc
      OvmfPkg/OvmfPkgIa32.dsc
      OvmfPkg/OvmfPkgIa32X64.dsc
      OvmfPkg/OvmfPkgX64.dsc
    
    All these platforms statically inherit PcdPciDisableBusEnumeration=FALSE
    from "MdeModulePkg.dec". Remove the the PCD and everything that depends on
    it from the PciHostBridgeLib instance. Namely, remove the logic that
    determines the root bridge apertures by (a) scanning the entire bus,
    device and function number space, and (b) parsing the BAR values that were
    pre-set by the Bhyve or Xen machinery.
    
    "XenSupport.c" used to be listed explicitly in "Maintainers.txt", remove
    it from that spot too.
    
    Cc: Andrew Fish <afish@apple.com>
    Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Leif Lindholm <leif@nuviainc.com>
    Cc: Michael D Kinney <michael.d.kinney@intel.com>
    Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>