devices: VPP crash when doing packet trace dump

devices: VPP crash when doing packet trace dump

VPP recycles the graph node index when the interface is deleted. Most tx graph
nodes have format_tx_function. tapcli does not. If tapcli happens to
recycle a tx graph node index which has format_tx_function, problem will arise
when showing the trace buffer which was collected for the node index that
tapcli recycles.

The fix is nothing fantasy, just add a format_tx_trace function which
does nothing in case it it called. tapcli is deprecated in 19.04. There is
no need to cherrypick this to later releases.

Type: fix
Ticket: VPP-1742

Signed-off-by: Steven Luong <[email protected]>
Change-Id: I010b3f35e59a474cb134a8b1bc718c242e3aa93c