Need help review a patch to enable network backend hint

Hi everyone,

I have made a PR on https://reviews.freebsd.org/D35143, that adds an optional option
to virtio-net frontend to allow users to explicitly choose the correct network backend.

Currently, bhyve is determining which network backend to use by the prefix of the interface,
which could cause some issues, for example when a user renamed a tap interface with a
name that does not start with "tap". This PR allows the user to put "type=tap" to hint that it is
a tap interface.

It will be great if someone can help review it and determine if it is acceptable to land.

Thanks,
Michael