utils.stacktrace: Allow passing logger as well as log name

Sometimes we already have the logger so it makes a little sense to pass
logger name only to obtain the logger again in this function. Lets allow
both.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/utils/stacktrace.py

@@ -32,17 +32,18 @@ def log_exc_info(exc_info, logger=''):
     Log exception info to logger_name.
 
     :param exc_info: Exception info produced by sys.exc_info()
-    :param logger: Name of the logger (defaults to '')
+    :param logger: Name or logger instance (defaults to '')
     """
-    log = logging.getLogger(logger)
-    log.error('')
+    if isinstance(logger, basestring):
+        logger = logging.getLogger(logger)
+    logger.error('')
     called_from = inspect.currentframe().f_back
-    log.error("Reproduced traceback from: %s:%s",
+    logger.error("Reproduced traceback from: %s:%s",
                  called_from.f_code.co_filename, called_from.f_lineno)
     for line in tb_info(exc_info):
         for l in line.splitlines():
-            log.error(l)
-    log.error('')
+            logger.error(l)
+    logger.error('')
 
 
 def log_message(message, logger=''):
@@ -50,11 +51,12 @@ def log_message(message, logger=''):
     Log message to logger.
 
     :param message: Message
-    :param logger: Name of the logger (defaults to '')
+    :param logger: Name or logger instance (defaults to '')
     """
-    log = logging.getLogger(logger)
+    if isinstance(logger, basestring):
+        logger = logging.getLogger(logger)
     for line in message.splitlines():
-        log.error(line)
+        logger.error(line)
 
 
 def analyze_unpickable_item(path_prefix, obj):