virt: Improve vm_xml device support + unittests

* Add ability to compare LibvirtXMLBase derived instances
* Fixed indentation problems in libvirt_xml.devices.librarian
* Added ability to create devices from ElementTree elements
* Added simplistic input element XML validation
* Simplified Serial device class hierarchy
* Added new unittests for serial devices
* Added new unittests for vm_xml device enumeration / comparison

Signed-off-by: Chris Evich <cevich@redhat.com>