PCI: Document reason for using pci_is_root_bus()

Some code assumes "bus->self == NULL" means the bus is a root bus.  This
adds a comment explaining why this is incorrect ("virtual" buses added for
SR-IOV have "bus->self == NULL" but are not root buses).

No functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>