vlib: set log tap level <level> does not work for some keywords

vlib: set log tap level <level> does not work for some keywords

While some levels such as debug and emerg work, others don't. See below.

DBGvpp# set log class tap level warn
set log class tap level warn
set logging class: unknown input `level warn'
DBGvpp# set log class tap level debug
set log class tap level debug
DBGvpp# set log class tap level info
set log class tap level info
set logging class: unknown input `level info'
DBGvpp# set log class tap level err
set log class tap level err
DBGvpp# set log class tap level crit
set log class tap level crit
set logging class: unknown input `level crit'
DBGvpp# set log class tap level emerg
set log class tap level emerg
DBGvpp#

Cause:
The reason for the failure for the shorter keywords is level_str is unformatted
with %v which is not null terminated. For example, the character after "info"
could be anything in level_str. The memcmp with size of the macro keyword __##uc
which includes the null character or 5 in this case and thus the comparison fails.

Fix:
Use %s which insure level_str is null terminated. Use strcmp to rule out
false positve match like "debugxxx" against keyword "debug".

Change-Id: I7a2d97a0f7f618df105da7eca791618dce04d21e
Signed-off-by: Steven <sluong@cisco.com>