qmp: add Monitor which uses cgo bindings to interact with libvirt

[mdlayher]

The libvirt-go cgo bindings will likely provide the best stability and error handling capabilities for interacting with libvirt, so we should add the ability to use them. They must be guarded by a build tag.

If built without the tag, all cgo operations will return error stubs.

[robertojrojas]

I've been looking into this issue and here is one idea on addressing the build tag guard.
I thought about creating 3 files like so:
libvirtgo.go (this file contains implementation methods entry point for Monitor, etc...
It will call the correct implementation method based on the GOOS where it's built)

libvirtgo_linux.go (this file contains the calls to libvirt-go package)

libvirtgo_others.go [build tag: // +build !linux](this file contains operations returning error stubs)
Of course, there will be *_test.go files with the corresponding tests.

Does this make sense?

[mdlayher]

Yep, seems reasonable to me. I'm happy as long as it compiles on all platforms, even if every method returns an error when not running on Linux.

[robertojrojas]

I'm getting closer to a PR for this issue. As for the dependency to the libvirt-go package, would Go 1.5+ vendoring suffice?

[mdlayher]

Since we are providing packages and not an application, we don't explicit vendor our dependencies. Importing it without vendoring is fine, especially because both repositories belong to DigitalOcean.