hw/pci: add pci_init_nic_devices(), pci_init_nic_in_slot()

hw/pci: add pci_init_nic_devices(), pci_init_nic_in_slot()

The loop over nd_table[] to add PCI NICs is repeated in quite a few
places. Add a helper function to do it.

Some platforms also try to instantiate a specific model in a specific
slot, to match the real hardware. Add pci_init_nic_in_slot() for that
purpose.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>