virlog: Introduce Type{To,From}String for virLogDestination

virlog: Introduce Type{To,From}String for virLogDestination

In order to refactor the ugly virLogParseOutputs method, this is a neat way of
finding out whether the destination type (in the form of a string) user
provided is a valid one. As a bonus, if it turns out it is valid, we get the
actual enum which will later be passed to any of virLogAddOutput methods right
away.