vty: log failed vty command

Add a log label argument to f_vty_wait_for_prompt(), and feed the sent
command from f_vty_transceive*(), so that the failure verdict already
lists the vty command that caused the failure.

A common error is to issue insufficient 'exit' commands, so that I often
think the newly added VTY command failed, even though it is a subsequent
command causing the failure. I want to shorten the "time-to-aha" there.

Change-Id: Icfd739db150d86e9256a224f12dc979dcd77879f
1 file changed