network_bandwidth: make test work only when guest is using default network.

When we start a test with ./run, we will get a netdst = user in test script.
This will break network_bandwidth test.

And the test of portgroup is only applicable for interface of network type.

The original thought about this test is assuming guest using default network.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>