virt: Fix a bug in check_iface func

    Command 'virsh iface-list --all' may output like this:
    eth1                 active     00:0c:29:7d:0a:36
    lo                   active     00:00:00:00:00:00
    test                 inactive
    virbr0               active     00:0c:29:7d:0a:2c
And the original source cannot get 'test' interface.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>