ixgbe: update if_sriov with ix-3.3.38 changes

There are some critical fixes here. The PF must communicate with each VF
slot (vf->pool), only VFs shall use 0 for everything.

IXGBE_FEATURE_SRIOV needs to be set before calling ixgbe_if_init().

With these changes, ixv(4) now attaches to VFs, but after bringing up
VFs, the PF and VF still are not correctly passing traffic.

MFC after: 1 week