Fixes for cable not connected in VirtualBox 4.3 (and 4.2)

I've only been able to verify the changes for 1.5.1 with VirtualBox 4.3 and host only adapters, and it fixed the issue (that's https://github.com/oscarrenalias/vagrant-1/commit/0314453f467ee94cae39f70bbcdd01f094405c77). 

The 3 remaining commits (for the rest of network adapters as well as VirtualBox 4.2.x) are only my guess so    feel free to merge/ignore as required.

Unit tests seemed to pass ok. I was not sure how to run the acceptance test suite so I was not able to verify that.