Do not inconvinence other VirtualBox DHCP servers

This PR is to continue to address Issue #92 but to not affect non Chef-BCPC VirtualBox networks.

In particular, this makes the code only:
* Delete DHCP servers which are on networks without VMs (Issue #92)
* Delete DHCP servers which are on networks explicitly provided (legacy non-Vagrant VM stand-up)

Also, while here, I removed a code block which claimed support for VirtualBox 4.2.2 (as ```-ifname``` works for me on 4.3.8 and we have removed support for VBox less than 4.3).