devices: create dummy device-input node to keep nexts in sync

devices: create dummy device-input node to keep nexts in sync

device-input feature arc requires that all input-nodes have
nexts in sync. packet-generator tends to call vlib_node_add_next
when new stream is created and that puts nexts out of sync.

With this change all input node are siblings of device-input
node so call to vlib_node_add_next(...) will install same next
to the whole family.

Change-Id: I33d79492e5f30f348af19e527f36fe0222c524d7
Signed-off-by: Damjan Marion <[email protected]>