Hi, I have same issue. I downgraded libvirt to 4.9.0-2 but similar error occurred:
601: error : virCgroupSetValueStr:480 : Unable to write to ‘/sys/fs/cgroup/blkio/machine.slice/machine-qemu\x2d1\x2dWhonix\x2dGateway.scope/blkio.weight’: No such file or directory
I tried to determine a suspicious package upgrade by seeing /var/log/pacman.log but it was difficult due to too many upgrades 
Most suspicious one would be:
[2019-03-18 14:19] [ALPM] upgraded linux (4.20.3.arch1-1 -> 5.0.2.arch1-1)