MFC r279445,r279464,r283670:

r279445 (by rstone):

Document pci_iov_attach/detach in pci.9

r279464 (by rstone):

Document the interface for defining a configuration schema

r283670 (by jhb):

Create a separate kobj interface for leaf-driver PCI IOV methods.

Leaf drivers should not import the PCI bus interface to add IOV handling.
Instead, move the IOV client methods to a separate kobj interface.