I’m running Linux Mint 18.3 which is based on Ubuntu 16.04 Xenial. Until yesterday I used the Personal Packet Archive ppa:jacob/virtualisation to get more up-to-date releases of Qemu, libvirt, and virt-manager.
Ubuntu and Linux Mint recently released security updates for their official (but old) qemu and libvirt packages to address the Meltdown and Spectre vulnerabilities. Unfortunately the Ubuntu 16.04 releases in the ppa:jacob/virtualisation archive have not been updated, judging from the upload date.
I decided to downgrade to the official releases for security reasons. Soon enough there will be Linux Mint 19 (based on Ubuntu 18.04), which features the newer versions of qemu and libvirt with the necessary security updates. Those using Ubuntu have already Ubuntu 18.04 at their disposal.
Downgrading qemu and libvirt unfortunately brought some performance degradation, be it because of the security updates that were implemented, or because of improvements in code in newer releases. At about 10% degradation (according to Passmark 8) it’s not earth-shattering, but still.