tests.netperf: using wait_for_get_address to get guest address

Some times when boot guest with two nics, the second nic can not
get ip address.
The wait_for_get_address will try to renew guest ip and try to get
guest ip via serial console. it is more safe than get_address

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