setup: Use xgettext --add-comments=translators

So in the code we can prefix comments with 'translators:' before
translated strings to have them show up in .pot file output

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

setup.py

@@ -660,6 +660,7 @@ class ExtractMessages(distutils.core.Command):
         potfile = "po/virt-manager.pot"
         xgettext_args = [
             "xgettext",
+            "--add-comments=translators",
             "--msgid-bugs-address=" + bug_address,
             "--package-name=virt-manager",
             "--output=" + potfile,

virtinst/xmlbuilder.py

@@ -454,8 +454,9 @@ class XMLBuilder(object):
         # know don't work
         forbid = [" "]
         if not val:
-            raise ValueError(
-                _("A name must be specified for the %s") % name_label)
+            # translators: value is a generic object type name
+            raise ValueError(_("A name must be specified for the %s") %
+                    name_label)
         for c in forbid:
             if c not in val:
                 continue