avocado.utils: Straighten up internal imports

It is nice to be able to refer to modules using the
dot notation. This fixes some missing imports in the
avocado.utils module, as well as it removes unused
ones.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/utils/__init__.py

@@ -12,9 +12,14 @@
 # Copyright: Red Hat Inc. 2013-2014
 # Author: Lucas Meneghel Rodrigues <lmr@redhat.com>
 
-import logging
-import random
-import string
+import archive
+import build
+import crypto
 import download
-import process
 import io
+import memory
+import network
+import params
+import process
+import remote
+import virt