vlib: fix vlib_log for elog

vlib: fix vlib_log for elog

test output before fix:
DBGvpp# event-logger clear
DBGvpp# test log warn cli log test-log for-elog
cli/log            [warn  ]: test-log for-elog
DBGvpp# test log info cli log test-log for-elog
cli/log            [info  ]: test-log for-elog
DBGvpp# show event-logger
2 of 131072 events in buffer, logger running
      53.022586433: log-notice: test-log for-elog
      60.318329361: log-debug: test-log for-elog
DBGvpp#

test output after fix:
DBGvpp# event-logger clear
DBGvpp# test log warn cli log test-log for-elog
cli/log            [warn  ]: test-log for-elog
DBGvpp# test log info cli log test-log for-elog
cli/log            [info  ]: test-log for-elog
DBGvpp# show event-logger
2 of 131072 events in buffer, logger running
      18.362721151: log-warn: test-log for-elog
      25.124570555: log-info: test-log for-elog
DBGvpp#

Type: fix

Signed-off-by: luoyaozu <luoyaozu@foxmail.com>
Change-Id: Ie1122787f9efb611cdafc671b4ccf68b43984924