Today we upgraded a server that ran a bunch of VMs to Fedora 25 and all the VMs failed to come back online after rebooting.
Checking the logs I found this:
2017-04-14T08:39:35.304547Z qemu-system-x86_64: Length mismatch: 0000:00:03.0/virtio-net-pci.rom: 0x10000 in != 0x20000: Invalid argument
2017-04-14T08:39:35.304579Z qemu-system-x86_64: error while loading state for instance 0x0 of device 'ram'
2017-04-14T08:39:35.304759Z qemu-system-x86_64: load of migration failed: Invalid argument
2017-04-14 08:39:35.305+0000: shutting down
After searching fruitlessly for all of those error messages in Google, I finally discovered the problem. The upgrade had shut libvirt down, which saved the state of all the machines into /var/lib/libvirt/qemu/save/. Then it started it back up and the save files were no longer compatible with the new qemu/libvirt binaries.
The solution was to just delete the save files in /var/lib/libvirt/qemu/save/ (which is a little sad, because it’s like yanking the power cord out of a running box). The VMs all started fine after that.