I think 'info-balloon' should be synchronous and without side-effects.
ie return the current stats that QEMU has. We could then add a separate
'refresh-balloon' command + async event notification when that completes.
The tool that is requiring the stats could thus refresh as often as it
likes.
This would work well for the QMP case, but what about for a traditional
monitor? We could include a sequence number or timestamp in the memory
stats results so the user could tell that they were updated. This
doesn't seem very user friendly though