Just ran the command from a commandline via SSH and it will give you more information about the failure:
/usr/bin/systemd-run --scope --slice qemu --unit 100 -p 'KillMode=none' -p 'CPUShares=1000' /usr/bin/kvm -id 100 -chardev 'socket,id=qmp,path=/var/run/qemu-server/100.qmp,server,nowait' -mon...