ixl/iavf(4): Change ixlv to iavf and update it to use iflib(9)

ixl/iavf(4): Change ixlv to iavf and update it to use iflib(9)

Finishes the conversion of the 40Gb Intel Ethernet drivers to iflib(9) for
FreeBSD 12.0, and fixes numerous bugs in both ixl(4) and iavf(4).

This commit also re-adds the VF driver to GENERIC since it now compiles and
functions.

The VF driver name was changed from ixlv(4) to iavf(4) because the VF driver is
now intended to be used with future products, not just with Fortville/Fort Park
VFs.

A man page update that documents these drivers is forthcoming in a separate
commit.

Reviewed by:    sbruno@, kbowling@
Tested by:      jeffrey.e.pieper@intel.com
Approved by:	re (gjb@)
Relnotes:       yes
Sponsored by:   Intel Corporation
Differential Revision: https://reviews.freebsd.org/D16429