Support different backend hypervisors

[mhewedy]

Currently, vermin working on VirtualBox as the only backend hypervisor.

This limitation was because works on its own native images. but now vermin supports vagrant boxes as of #11 .

So I think vermin now can support many hypervisors without the concern of image availability.

One of these hypervisors I intent to support is Hyper-V

Steps:

1. decouple current implementation from virtualbox #29
2. support only vagrant images #31
3. change directory structure to support multiple hypervisors #32