Paolo,
commit 41fc57e44ed64cd4ab5393d83624afd897dabd4f trace: split trace_init_file out of trace_init_backends causes hangs for libvirt when checking the daemonized process, if the qemu was build with trace-backend=simple. Also calling qemu-system-s390x -machine accel=kvm -S -no-user-config -nodefaults -nographic -M none -qmp unix:/var/lib/libvirt/qemu/capabilities.monitor.sock,server,nowait -pidfile /var/lib/libvirt/qemu/capabilities.pidfile -daemonize just hangs without "trace-backend=simple" things are fine.