If a guest has multiple network devices with the same MAC address,
when we online update the second device, libvirtd always updates
the first one.

commit def31e4c forgot to fix the online updating scenario. We need to
use virDomainNetFindIdx() to find the correct network device.
Signed-off-by: NZhou Yimin <zhouyimin@huawei.com>
Signed-off-by: NZhang Bo <oscar.zhangbo@huawei.com>