qmp: allow NewLibvirtMonitor to utilize multiple libvirt "drivers"

[mdlayher]

In order to avoid exposing too many options for qmp.Monitor types that interact with libvirt, we may be able to mask them all behind a new parameter in the qmp.NewLibvirtMonitor constructor.

Currently, the constructor looks like:

func NewLibvirtMonitor(uri, domain string) (*Libvirt, error)

The proposed constructor could be something like:

func NewLibvirtMonitor(driver LibvirtDriver, uri, domain string) (*Libvirt, error) {

Where driver could be one of the following:

type LibvirtDriver int

const (
    LibvirtDriverShell LibvirtDriver = iota
    LibvirtDriverRPC   LibvirtDriver
    LibvirtDriverCgo   LibvirtDriver
)

The shell driver is already implemented, and the native RPC driver is well underway. The cgo driver could be added in a future PR, and guarded with build tags to make it a no-op unless compiled with the correct tags.

[mdlayher]

This probably doesn't make much sense anymore since the shell and RPC drivers have totally different signatures. Closing.