MFC r270332:
r270332 (by royger):

pci: add a new pci_child_added newbus method.

This is needed so when running under Xen the calls to pci_child_added
can be intercepted and a custom Xen method can be used to register
those devices with Xen. This should not include any functional
change, since the Xen implementation will be added in a following
patch and the native implementation is a noop.

Sponsored by: Citrix Systems R&D
Reviewed by: jhb

dev/pci/pci.c:
dev/pci/pci_if.m:
dev/pci/pci_private.h:
dev/pci/pcivar.h:

• Add the pci_child_added newbus method.