VM start fails with this error:
Running: create_domain_xml LibVirt: Call to virDomainCreateXML failed: XML error: invalid maximum number of vCPUs '0'
Run 'service libvirtd restart' on the hypervisor where the vm failed to start.
If that does not fix the problem, you can also cold migrate the vm to another HV and start it there.
Libvirt may have started with incorrect parameters, or there may be too many virtual machines currently running on the hypervisor.