Reject network devices which appear to be duplicates of those already
available via a different underlying hardware device.  On a Xen PV-HVM
system, this allows us to filter out the emulated PCI NICs (which
would otherwise appear alongside the netfront NICs).

Note that we cannot use the Xen facility to "unplug" the emulated PCI
NICs, since there is no guarantee that the OS we subsequently load
will have a native netfront driver.

We permit devices with the same MAC address if they are attached to
the same underlying hardware device (e.g. VLAN devices).

Inspired-by: Marin Hannache <git@mareo.fr>
Signed-off-by: Michael Brown <mcb30@ipxe.org>