igb: Change how we check for pre-existing and assigned VFs

Adapt the pre-existing and assigned VFs code to the ixgbe way introduced
in commit 9297127b.

Instead of searching the enabled VFs we use pci_num_vf to determine enabled VFs.
By comparing to which PF an assigned VF is owned it's possible to decide
whether to leave it enabled or not.

Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Robert Garrett <robertx.e.garrett@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>