NixOps-managed network resource for virtualbox backend

[onixie]

@AmineChikhaoui @domenkozar @edolstra

I'm working on a fork of nixops-vbox plugin to enable network resource management for the virtualbox backend.

The motivation is to solve a bunch of long-open issues originated from the same cause, IMO, due to the lack of network capability, and to provide user more options and variations regarding to the network configuration of the machine.

The basic idea is

1. Add a vbox-network resource and extend the backend so that it can manage a set of the resources. These resources can be incarnated as either Host-only or NAT network but with similar layout of the IP addresses in VirtualBox. It can be thought as mimicing the vpc resource in gce or aws backend.
2. For VirtualBox >6.1, since we can finally bind a fixed IP address to a machine using the default DHCP server in VirtualBox. We can add a new staticIPs option under the vbox-network resource and extend the backend so that it can manage those static IPs using the new VirtualBox feature.
3. Also add a networks option under deployment.virtualbox and allow user specify the networks for the machine. This can be thought as a generalization of current hard-coded NAT + vboxnet0 configuration. In order to still keep the connectivity, we can enforce that at least one Host-only network must be specified.

I haven't yet finalized the PR but I would like to hear about your opinions and reviews before progressing further. My fork can be found at: onixie/nixops-vbox@81c21f3

Hopefully, this can solve
#202 (an obsolete? PR addressing similar concern)
#286 (when using VirtualBox > 6.1)
#411
#978