libvirt.tests: Add virsh nwfilter group tests

This is part of PR 1194 which belongs to test provider libvirt.

The virt-test part of PR 1194 remake is merged:
https://github.com/autotest/virt-test/pull/1410

Added test case for virsh filter group:
\# 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