Guest and Host IP address are ignored when forwarding ports

[Johannes Maria Frank]

When forwarding ports, neither guest nor host IP are considered.
Example from Vagrantfile:

config.vm.network "forwarded_port", guest: 8080, guest_ip: "192.168.5.100", host: 8080, host_ip: "127.0.0.1"

Although specified, guest_ip (and host_ip) are ignored.

[Johannes Maria Frank]

I already have a fix for this working on my local repo, but with test suite and port forwarding currently broken, it will be difficult to add another PR without creating a mess. Please consider merging #396 and #397 first.
Thank you.