Re: [libvirt-users] Is there a way for telling libvirt to auto define an xml under /etc/libvirt/qemu?

Yes, 'virsh define <xml>' on each node is the documented and supported way to maintain a persistent definition of the same domain across multiple hosts. Or you may want to look into VDSM (part of oVirt, and available in Red Hat's RHEV product), which manages definitions of domains across multiple hosts by using a common store managed by VDSM and only using transient guests when using libvirt on any one host in the managed cluster.