tcd should not hardcode ifb number "ifb0"

https://github.com/kinvolk/tcd/blob/c49ccf8d79859414a8041f4feb9032386068f450/lib/dbus.go#L138:

> ```
>   "ip link add ifb0 type ifb",
>   "ip link set ifb0 up",
> ```

By default, the ifb kernel module creates ifb0 and ifb1. So tcd should not reuse them but get a number dynamically.

As a workaround, ifb can be loaded without automatically creating devices:

```
insmod ifb numifbs=0
```
