bridges can be renamed, and if so, things break

[jmgurney]

I have renamed a bridge to guestbrg0, and it fails to pick up that this bridge exists. This is because iohyve assumes that all bridges are named bridge[0-9]+. This patch adapts to using groups, and, as you cannot create a renamed bridge (that is if you rename bridge2 to guestbrg0, you can no longer create bridge2), the new bridge detection logic needs to be changed. It turns out, that at least w/ 11.3, you can simply do ifconfig bridge create, and ifconfig will print the name of the new bridge. I have verified that the patch handles when a bridge exists, and also creates a new bridge as needed.
ioh-setup.txt