Use slot number to report MAC address

[jarrpa]

Use a network device's slot number to detect and store the generated MAC address if one hasn't already been provided.

This accommodates scenarios where multiple devices are defined on the same network.

Closes: #645

[electrofelix]

Conceptually this works but it relies on the order of the devices returned from listing vagrant networking interfaces must match the order created in the libvirt XML. I believe that vagrant-libvirt supports setting :libvirt__adapter to insert declared devices in an order different to that they were declared in the vagrant definition thus violating that assumption and meaning we cannot rely on the index returned from interating the network interfaces returned by vagrant matching the position in the XML defined.

Some tests would confirm this either way, which lack of time to write was blocking me from doing anything one issue.

[jarrpa]

....if so, then this seems extremely broken. Is it possible to create a domain with two devices and number one of them as adapter 3? Then you'd have an empty slot between, it would seem, and that should cause an error when trying to iterate through each_with_index. Unless I'm still too new to Ruby.

[pronix]

this patch will broke existed behaviour, please update it.

[electrofelix]

@jarrpa agreed leaving a hole seems incorrect but when you have multiple networks defined the ability to explicitly control device and network order is supported so need to retain it while fixing here.

Perhaps it's not sufficiently useful that it could be deprecated and removed in the future, but would have to go through a period where users are notified of the plan to remove support.

[jarrpa]

@pronix @electrofelix Okay, changed it up a bit, going off the comment made here. Also cleaned up a little bit of logic to make the change easier to read.