virttest : add a netperf class to support netperf test

This patch add a class of netperf, make netperf test easily.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
qemu.test.migrate_with_netperf : refactor the code base utils_netperf

This patch refactor the code base utils_netperf

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