After upgrading qemu to 1:10.1.0+ds-1 (from 1:10.0.3+ds-4), starting virtual machines with "virsh start" fails, and virt-manager cannot connect to libvirtd. Downgrading the qemu packages to 1:10.0.3+ds-4 fixes the problem.
Sorry, I just realized this is most likely a duplicate of #1112273 Have a nice day,
https://bugs.debian.org/1112330 (CC'd). FWIW. Thanks, /mjt
Let's close this bug report, it is a duplicate indeed, but I fail to merge it with 1112273. /mjt