iavf(4): Finish rename/rebrand internally

Rename functions and variables from ixlv to iavf to match the
user-facing name change. There shouldn't be any functional changes
with this change, but this may help with browsing the source code
and reducing diffs in the future.

Submitted by: kbowling@
Reviewed by: erj@, sbruno@
Approved by: re (gjb@)
Differential Revision: https://reviews.freebsd.org/D17544