yamlgraph: Refactor parsing for dynamic resource registration

Avoid use of the reflect package, and use an extensible list of registred
resource kinds. This also has the benefit of removing the empty VirtRes and
AugeasRes struct types when compiling without libvirt and libaugeas.

resources/exec.go

@@ -34,6 +34,7 @@ import (
 
 func init() {
 	gob.Register(&ExecRes{})
+	RegisterResource("exec", func() Res { return &ExecRes{} })
 }
 
 // ExecRes is an exec resource for running commands.