PCI: Create SR-IOV virtfn/physfn links before attaching driver

When creating virtual functions, create the "virtfn%u" and "physfn" links
in sysfs *before* attaching the driver instead of after.  When we attach
the driver to the new virtual network interface first, there is a race when
the driver attaches to the new sends out an "add" udev event, and the
network interface naming software (biosdevname or systemd, for example)
tries to look at these links.

Signed-off-by: Stuart Hayes <stuart.w.hayes@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>