DataSize: define __slot__

For small classes, like this one, with known attributes, using dict
wastes a lot of RAM. Defining __slots__ as a list will save a lot of
memory, while improving the performance a bit.

Before this patch:

>>> timeit.timeit('data_structures.DataSize("1M").b',
                  setup='from avocado.utils import data_structures',
                  number=1000000)
>>> 1.1383171081542969

After this patch:

>>> timeit.timeit('data_structures.DataSize("1M").b',
                  setup='from avocado.utils import data_structures',
                  number=1000000)
>>> 1.1071600914001465
Signed-off-by: NAmador Pahim <apahim@redhat.com>

avocado/utils/data_structures.py

@@ -266,6 +266,7 @@ class DataSize(object):
                                ('m', 1048576),  # 2**20
                                ('g', 1073741824),  # 2**30
                                ('t', 1099511627776)])  # 2**40
+    __slots__ = ['_value', '_unit']
 
     def __init__(self, data):
         try: