Fix undefined args access for x86_64.

From libc manual for vsnprintf:
   The  functions  vprintf(),  vfprintf(), vsprintf(), vsnprintf()
   are equivalent to the functions printf(), fprintf(), sprintf(), snprintf(),
   respectively, except that they are called with a va_list instead of a
   variable number of arguments.  These  functions  do  not  call  the  va_end  macro.
   Because they invoke the va_arg macro, the value of ap is undefined after the call.

We need to allocate/end new va_list for each vsnprintf.

Change-Id: I66ec058033be1cb918e7b2bc84ca546800da226b
Signed-off-by: Fengwei Yin <fengwei.yin@intel.com>