Virtnet reduction [DRAFT 3]

These commits simplify (IMHO) ``utils_net.VirtNet`` and related classes.  I think it's a much clearer division of responsibilities and unambiguous behaviors.  Rather than try and impose relationships upon callers, these changes empower callers by simply providing classes which can work well together.  It also reorganizes a lot of qemu-kvm virt-test specific functionality under a single class, close to related data for consumption.  This permits proper unittesting, whereas prior, it was virtually impossible due to impracticality of unitests for the VM classes.

**DRAFT #3**:  All default test-sets working, specialized tests still need verifying.

I'd appreciate feedback on any change or test that seems broken.  Thanks!