tests.cfg.netperf.cfg : Make test support different types of network adapters

Remove the limit of virtio_net, make the test support different types of
network adapters like e1000 and rtl8139

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Merge pull request #741 from humanux/netperf

 tests.cfg.netperf.cfg : Make test support different types of network apapters