app/testpmd: send failure logs to stderr

app/testpmd: send failure logs to stderr

Running with stdout suppressed or redirected for further processing
is very confusing in the case of errors. Fix it by logging errors and
warnings to stderr.

Since lines with log messages are touched anyway concatenate split
format strings to make it easier to search using grep.

Fix indent of format string arguments.

Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Xiaoyun Li <xiaoyun.li@intel.com>