ixv: Separate VFTA table for each interface

The vlan setting is independent for each interface. Use VFTA table in
'struct ixgbe_softc' that is already defined.

This pull request fixes following bug scenario.

create ixv0.10
create ixv1.10
destroy ixv1.10
create ixv0.11
ixv0.10 no longer receives vlan 10 packets.

In this case, destroying ixv1.10 affects to ixv0.

Pull Request: https://github.com/freebsd/freebsd-src/pull/774

(cherry picked from commit 61a8231d152c38ac96b697ea3ca8ff220f3aa536)