qemu: snapshot: Save persistent domain config when taking external snapshot

qemu: snapshot: Save persistent domain config when taking external snapshot

Commit 55bbb011b965c7962933604c70f61cef45e8ec04 introduced a regression
where we forgot to save the persistent domain configuration after an
external snapshot. This would make libvirt forget the snapshots and
effectively revert to the previous state in the following scenario:

1) Start VM
2) Take snapshot
3) Destroy VM
4) Restart libvirtd

Also fix spurious blank line added by patch mentioned above.