Add the PCI HDAudio device model from the 2016 GSoC. Detailed information
can be found at

https://wiki.freebsd.org/SummerOfCode2016/HDAudioEmulationForBhyve

This commit has evolved from the original work to include Capsicum
integration. As part of that, it only opens the host audio devices once
and leaves them open, instead of opening and closing them on each guest
access. Thanks to Peter Grehan and Marcelo Araujo for their help in
bringing the work forward and providing some of the final techncial push.

Submitted by: Alex Teaca <iateaca@freebsd.org>
Differential Revision: D7840, D12419