Make bus types enum constants instead of strings

[jaypipes]

The bus types are strings with a mix of uppercase and lowercase runes. Make them constant enums instead.

[jaypipes]

@jpo-joyent @yrobla @pwFoo @blp1526 would you all mind doing a quick review of this patch please?

#109

If you are using ghw as a library (versus the ghwc CLI tool) and are referencing the ghw.Disk.BusType field, this patch will end up breaking your code if you haven't pinned your ghw dependency to 0.2 release or prior. Can you let me know if you're OK with this? ghw is still in a fluid pre-1.0 release state and I'm trying to clean things like this up before the 1.0 release.

Thank you!
-jay

[yrobla]

Change is ok for me . We are not yet using library (although we want to) , but using ghwc CLI tool.

[blp1526]

LGTM.

(This Makefile has make lint, and it warns us "don't use ALL_CAPS in Go names; use CamelCase (golint)" at bus_type.go. If ghw keeps using gometalinter, I think that it is better to modify the warning lines.)

[jaypipes]

Yeah, I'm not a fan of that particular Go-ism. Maybe it's my C background but I just like having constants use ALL_CAPS. So, that's not going to change... I might just remove the warning lines for that.

[jpo-joyent]

Yep, broke my code but was trivial to fix. Thank you for giving us a heads up! :)

[jpo-joyent]

Actually no, this breaks macOS again, and in a way that I can not easily fix.

In macOS, the bus type is a string that comes from the IOKit BusProtocol field, whose legal values are not documented, and whose source is not available. The only value I've seen is PCI-Express, but this does not mean other things are not possible, just that I have not seen them so far. BusType being a string let me just pass them through and let the caller have something useful in cases I have not anticipated, but now I don't have a clean option. Always pass BUS_TYPE_UNKNOWN? That seems less than ideal...