Recently I encountered this issue in OVMM 3.2.9 while starting a vm with
xm create <vm.cfg path>
The reason behind this found was : the vm was not shutdown properly and the lock file is still there even if VM is down.
So the places to look at :
Look at the log file and if the lock file is present under /var/run/ovs-agent/ with the id of the vm which is not starting, just delete the lock file and then VM will start successfully.