OvmfPkg/QemuBootOrderLib: clean up translation of virtio-net over MMIO
The "/MAC(" suffix of the translated UEFI devpath prefix is unnecessary
for matching, because the virtio-mmio base address in VenHwString is
unique anyway. Furthermore, the partial string "MAC(" cannot be processed
by ConvertTextToDevicePath(), which will become relevant later in this
series. Remove "/MAC(".