virtio-balloon crashes in a object assert when querying stats
Description of problem:
Fetch virtio-balloon stats will crash a QEMU crash with assert failures
Steps to reproduce:
1. ./qemu-system-x86_64 -device virtio-balloon,id=balloon -qmp qmp.sock
2. Connect to qmp.sock
3. Issue  'qom-get path=/machine/peripheral/balloon property=guest-stats'
4. QEMU go boom!
Additional information:
This is a regression caused by commit 0d2eeef77a33315187df8519491a900bde4a3d83, which failed to update `balloon_stat_names` with the new stats names, causing code to try to add a QDict entry with a NULL key.