plugins.diff: fixes diff plugin to get multiple variants

Since we have one variant per suite now, this plugin needs to get the
list of variants instead of only one.
Signed-off-by: NBeraldo Leal <bleal@redhat.com>

avocado/core/varianter.py

@@ -333,14 +333,20 @@ class Varianter:
     @classmethod
     def from_resultsdir(cls, resultsdir):
         """
-        Retrieves the job variants object from the results directory.
+        Retrieves the job variants objects from the results directory.
+
+        This will return a list of variants since a Job can have multiple
+        suites and the variants is per suite.
         """
         path = os.path.join(resultsdir, 'jobdata', VARIANTS_FILENAME)
         if not os.path.exists(path):
             return None
 
+        variants = []
         with open(path, 'r') as variants_file:
-            return cls(state=json.load(variants_file))
+            for variant in json.load(variants_file):
+                variants.append(cls(state=variant))
+        return variants
 
     def __len__(self):
         return self._no_variants

avocado/plugins/diff.py

@@ -390,7 +390,8 @@ class Diff(CLICmd):
         results = []
         variants = Varianter.from_resultsdir(resultsdir)
         if variants is not None:
-            results.extend(variants.to_str(variants=2).splitlines())
+            for variant in variants:
+                results.extend(variant.to_str(variants=2).splitlines())
         else:
             results.append('Not found\n')