Output arrays of files rather than a single file

This is needed because the version upgrade system now allows for a version upgrader to output any number of files.

Contributes to issue CURA-844.

plugins/VersionUpgrade/VersionUpgrade21to22/MachineInstance.py

@@ -97,4 +97,4 @@ class MachineInstance:
 
         output = io.StringIO()
         config.write(output)
-        return self._filename, output.getvalue()
+        return [self._filename], [output.getvalue()]

plugins/VersionUpgrade/VersionUpgrade21to22/Preferences.py

@@ -77,4 +77,4 @@ class Preferences:
         #Output the result as a string.
         output = io.StringIO()
         self._config.write(output)
-        return self._filename, output.getvalue()
+        return [self._filename], [output.getvalue()]

plugins/VersionUpgrade/VersionUpgrade21to22/Profile.py

@@ -130,4 +130,4 @@ class Profile:
 
         output = io.StringIO()
         config.write(output)
-        return self._filename, output.getvalue()
+        return [self._filename], [output.getvalue()]