multiplexer: Set root node name to empty string.

Get rid of "/root" for the root node and use empty string as name,
in order to avoid to prefix filters and to display information
with the /root prefix in every output.

The output will be just like this:
<pre>
./scripts/avocado multiplex mux.yaml --filter-only /hw/cpu/arm --filter-out /os
Variants generated:
Variant 1:    /hw/cpu/arm, /hw/disk/scsi, /env/debug
Variant 2:    /hw/cpu/arm, /hw/disk/scsi, /env/prod
Variant 3:    /hw/cpu/arm, /hw/disk/virtio, /env/debug
Variant 4:    /hw/cpu/arm, /hw/disk/virtio, /env/prod
</pre>

Signed-off-by: Ruda Moura <rmoura@redhat.com>