OVS: add the check for exists tap device
OVS: add the check for exists tap device

Check whether there is already a device with the same name has already
been attached to the ovs bridge or not. If so, skip the follow attaching
operation.

Signed-off-by: Hu Keping <ce660952c949ad2751377b45317337be332e80ac@huawei.com>

laijs/runv,gao-feng/runv,jinfenw/runv,wrouesnel/runv,gnawux/runv,feiskyer/runv,gnawux/runv,gnawux/runv,bergwolf/runv,gao-feng/runv,hyperhq/runv,wrouesnel/runv,laijs/runv,jinfenw/runv,bergwolf/runv,laijs/runv,hyperhq/runv,hyperhq/runv,bergwolf/runv,feiskyer/runv,jinfenw/runv,gao-feng/runv