I can confirm that downgrading libvirt and almost all related packages to version 10.3.0-3 restores the usual behaviour.
- Bug#1072723: libvirt0: qemu:///session not connected after log... Michael Ott
- Bug#1072723: libvirt0: qemu:///session not connected afte... Francesco C
- Bug#1072723: libvirt0: qemu:///session not connected afte... Francesco C
- Bug#1072723: libvirt0: qemu:///session not connected afte... Francesco C

