virt.test.nic_promisc : send command safely

Some times the kernel or other subsystems might print some info,
that will interfere with aexpect following command prompts and make
this case failed like cmd timeout.
This patch tried to resolve them by creating wrappers

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Merge pull request #687 from humanux/nic_promisc

virt.test.nic_promisc : send command safely