[pxeprefix] Display PCI vendor and device ID in PXE startup banner

[pxeprefix] Display PCI vendor and device ID in PXE startup banner

In the case of a misbehaving PXE stack, it is often useful to know the
PCI vendor and device IDs (e.g. for adding the device to the list of
devices with known broken support for generating interrupts).

The PCI vendor and device ID is already available to the prefix code,
and so can trivially be printed out.  Add this information to the PXE
prefix startup banner.

Signed-off-by: Michael Brown <mcb30@ipxe.org>