Extend ui_message_handler to always print xml

If JSON or plain text UIs are enabled, convert the data into a suitable
format before printing as usual

Author: thk123

src/util/ui_message.cpp

@@ -12,6 +12,7 @@ Author: Daniel Kroening, [email protected]
 #include <iostream>
 
 #include "cmdline.h"
+#include "convert_xml_json.h"
 #include "json_irep.h"
 #include "json_stream.h"
 #include "make_unique.h"
@@ -166,16 +167,22 @@ void ui_message_handlert::print(
     switch(get_ui())
     {
     case uit::PLAIN:
-      INVARIANT(false, "Cannot print xml data on PLAIN UI");
+    {
+      print(level, xml_to_pretty(data));
       break;
+    }
     case uit::XML_UI:
+    {
       out << data << '\n';
       flush(level);
       break;
+    }
     case uit::JSON_UI:
-      INVARIANT(false, "Cannot print xml data on JSON UI");
+    {
+      print(level, from_xml(data));
       break;
     }
+    }
   }
 }