igb: Enable SR-IOV configuration via PCI sysfs interface

Implement callback in the driver for the new PCI bus driver
interface that allows the user to enable/disable SR-IOV
virtual functions in a device via the sysfs interface.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>