Do not overwrite previously set options.

If user doesn't provide options from commandline during rebuild, such
option will be overwritten by those stored in VM XML, even if user
change such option in config file. This commit fixing that.

box.py

@@ -273,7 +273,7 @@ class Config:
         # the command line
         vm_info = vbox.get_vm_info() if vbox else {}
         for attr in self.ATTRS:
-            val = getattr(args, attr, None) or vm_info.get(attr)
+            val = getattr(args, attr, None)
             if not val:
                 continue
             setattr(self, attr, str(val))