virsh.iface_bridge: add a negative case for iface not existing.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
virsh.iface_bridge: fix bug when delay is not zero.

    when option of delay is not zero, we will get a FAIL in test.
    Because the default ping count is 3, so it will fail to ping
    remote_ip in 3 second.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #331 from yangdongsheng/iface_bridge

Add more case for ifc-bridge test/