OvmfPkg: QemuBootOrderLib: introduce ExtraRootBusMap

OvmfPkg: QemuBootOrderLib: introduce ExtraRootBusMap

SeaBIOS requires the OpenFirmware device paths exported in the "bootorder"
fw-cfg file to refer to extra (PXB) root buses by their relative positions
(in increasing bus number order) rather than by actual bus numbers.

However, OVMF's PCI host bridge / root bridge driver creates PciRoot(UID)
device path nodes for extra PCI root buses with UID=bus_nr, not position.
(These ACPI devpath UID values must, and do, match the UID values exposed
in QEMU's ACPI payload, generated for PXB root buses.)

Therefore the boot order matching logic will have to map extra root bus
positions to bus numbers. Add a small group of utility functions to help
with that.

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>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17964 6f19259b-4bc3-4df7-8a09-765794883524