virttest.utils_net : modify the method of get_linux_ifname

This patch modify the method of get_linux_ifname
we can get the all the nic interface in the guest or
only get the nic interface with mac address we assigned.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Merge pull request #686 from humanux/utils_net

 virttest.utils_net : modify the method of get_linux_ifname