ofw_pcibus: Inherit PF locality for SR-IOV VFs

[kbowling]

Summary

PCI VFs are allocated dynamically and have no corresponding OFW node.
The zero-filled OFW PCI devinfo currently leaves obd_node as 0, which
is not the invalid-node sentinel and can send NUMA lookup through an
unrelated firmware node.

Initialize dynamically allocated devinfo with an invalid OFW node. For
VF locality queries, use the owning PF's node when it exists. Fall back
to the PCI bus when neither the VF nor PF has a firmware node.

This preserves existing CPU-locality behavior for ordinary PCI devices
while making VF domain and interrupt placement follow their PF.

MFC after:      2 weeks