virttest.test_setup: changed forward delay to 4

Currently our script set bridge forward to 0, it cann't work
in some os. checking bridge doc 802.1D-1998 and latest
802.1D-2004, in these docs Bridege Forward Delay range time
is 4.0-30.0 seconds. so modify the value to 4

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>