tests: test_dist: Add 'appstream-util validate' check

And fix some issues it reports

Signed-off-by: Cole Robinson <crobinso@redhat.com>

data/virt-manager.appdata.xml.in

@@ -39,4 +39,9 @@
     <keyword>qemu</keyword>
     <keyword>kvm</keyword>
   </keywords>
+  <content_rating type="oars-1.1"/>
+  <releases>
+    <release version="2.2.1" date="2019-07-03"/>
+    <release version="2.2.0" date="2019-06-17"/>
+  </releases>
 </component>

tests/test_dist.py

@@ -119,3 +119,8 @@ def test_ui_translatable_atknames():
             (atkstr, atkstr))
     err += " ".join(failures)
     raise AssertionError(err)
+
+
+def test_appstream_validate():
+    subprocess.check_call(["appstream-util", "validate",
+        "data/virt-manager.appdata.xml.in"])