Add Nodedev_XML into libvirt_xml

cover-letter:
  bvirt_xml-accessor: modify class XMLElementInt.
  Add unittest for new XMLElementInt.
  virsh: Add nodedev_dumpxml in virsh modeule.
  virt: add nodedev_xml into libvirt_xml module.
  Add unittest for nodedev_xml.

 virttest/libvirt_xml/accessors.py   |   31 ++--
 virttest/libvirt_xml/nodedev_xml.py |  296 +++++++++++++++++++++++++++++++++++
 virttest/libvirt_xml_unittest.py    |  120 +++++++++++++-
 virttest/virsh.py                   |   24 +++
 4 files changed, 461 insertions(+), 10 deletions(-)
 create mode 100644 virttest/libvirt_xml/nodedev_xml.py