utils.vmimage: Use set to store providers

The IMAGE_PROVIDERS stores possible classes to provide images, let's
avoid unnecessary multiple occurrences.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/utils/vmimage.py

@@ -351,10 +351,10 @@ def list_providers():
     """
     List the available Image Providers
     """
-    return [_ for _ in globals().itervalues()
-            if (_ != ImageProviderBase and
-                isinstance(_, type) and
-                issubclass(_, ImageProviderBase))]
+    return set(_ for _ in globals().itervalues()
+               if (_ != ImageProviderBase and
+                   isinstance(_, type) and
+                   issubclass(_, ImageProviderBase)))
 
 
 #: List of available providers classes

docs/source/utils/vmimage.rst

@@ -97,7 +97,7 @@ with::
     class MyTest(Test):
 
         def setUp(self):
-            vmimage.IMAGE_PROVIDERS.append(MyProvider)
+            vmimage.IMAGE_PROVIDERS.add(MyProvider)
             image = vmimage.get('MyDistro')
             ...