ixgbe: Implement PCI SR-IOV sysfs callback operation

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

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
CC: Don Dutile <ddutile@redhat.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>