avocado.core.log: Initialize "root" logger

Initialize "root" logger as "NullHandler" to prevent other libraries
from initializing. Avocado adds file or stream handlers when it
requires them (avocado-vt).
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/log.py

@@ -70,6 +70,9 @@ DEFAULT_LOGGING = {
         },
     },
     'loggers': {
+        '': {
+            'handlers': ['null']
+        },
         'avocado': {
             'handlers': ['console'],
         },