At present, pci_nic_init() and pci_nic_init_nofail() assume that they will
only create a NIC under the primary PCI root.  As we add support for
multiple PCI roots, that may no longer be the case.  This patch adds a root
bus parameter to pci_nic_init() (and updates callers accordingly) to allow
the machine init code using it to specify the right PCI root for NICs
created by old-style -net nic parameters.  NICs created new-style, with
-device can of course be put anywhere.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>