avocado.plugins.vm: Provide better debug info on failures to load JSON

Provide the full output of the avocado execution, so we
understand better what was the actual failure.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/plugins/vm.py

@@ -52,7 +52,12 @@ class VMTestRunner(TestRunner):
         """
         avocado_cmd = 'avocado --json run --archive "%s"' % urls
         stdout = self.result.vm.remote.run(avocado_cmd)
-        results = json.loads(stdout)
+        try:
+            results = json.loads(stdout)
+        except Exception, details:
+            raise ValueError('Error loading JSON '
+                             '(full output below): %s\n"""\n%s\n"""' %
+                             (details, stdout))
         return results
 
     def run(self, params_list):