libvirt: Add nwfilter group command functions, xml modules and test cases

This patch set is for adding virsh nwfilter group xml modules, functions and basic cases.

  - libvirt.tests: Add virsh nwfilter define case
  All following five commands are wrapped in virsh.py
  \# virsh help filter
   Network Filter (help keyword 'filter'):
      nwfilter-define                define or update a network filter from an XML file
      nwfilter-dumpxml               network filter information in XML
      nwfilter-edit                  edit XML configuration for a network filter
      nwfilter-list                  list network filters
      nwfilter-undefine              undefine a network filter

  - libvirt.test: Add virsh nwfilter-dumpxml test
  This commit add virsh dumpxml test case and cfg

  - virttest.libvirt_xml.devices.librarian: Add network filter types
  This commit add network filter protocol types in the librarian type list

  - virttest.libvirt_xml.devices: Add network filter rule protocol modules
  This commit for adding 24 sub protocol type xml modules under devices

  - virttest.libvirt_xml: Add nwfilter xml module
  This is the main nwfilter xml module

  - libvirt.tests: Add virsh nwfilter define case
  This commit add nwfilter define test case and cfgs, which use the nwfilter xml module and protocol type sub xml modules for define filter.