libvirt.test: Add virsh nwfilter-dumpxml test

V1:
Simply dump network filter xml with normal and error test.

V2:
Only accept filter name from cfg as filter uuid may differ on
different host.
For check the dumpxml result, extract uuid and name from output
and compare with nwfilter-list to confirm.

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