libvirt_xml.devices device xml stubs and unittests

Unless all the class representations for each XML device are written, the entire libvirt_xml.vm_xml.VMXML.devices interface would continuously raise non-obvious exceptions.  This request implements stub-classes for every device type which represent the absolute minimum libvirt_xml.devices interface.  This allows VMXML.devices to properly represent an enumeration of all XML defined devices, even if some/most of those definitions is incomplete.