iavf(4): Split source and update to 3.0.26-k

The iavf(4) driver now uses a different source base from ixl(4), since
it will be the standard VF driver for new Intel Ethernet products going
forward, including ice(4). It continues to use the iflib framework
for network drivers.

Since it now uses a different source code base, this commit adds a new
sys/dev/iavf entry, but it re-uses the existing module name so no
configuration changes are necessary.

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

Reviewed by: kbowling@
Tested by: lukasz.szczepaniak@intel.com
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D28636