utils_test.libvirt: add a dargs to check_iface method.

This patch add a dargs to check_iface, then we can pass
    count and timeout to ping().

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #1842 from yangdongsheng/ping

utils_test.libvirt: add a dargs to check_iface method.