libvirt.tests: add virsh nwfilter list test

The test simply list filters and check the filter cfg file exist
under network filter config path

Signed-off-by: Wayne Sun <gsun@redhat.com>
libvirt.tests: Add nwfilter-undefine test case

Undefine network filter using given filter name or uuid in cfg.
If the filter is valid, backup filter xml before undefine and
define the filter back after undefine succeed.

Signed-off-by: Wayne Sun <gsun@redhat.com>