libvirt will not prevent two hosts running the same guest.

Situation:
1. You have a guest on node1
2. You migrate the guest to node2
3. The status according to virsh is:
 node1:guest shut off
 node2:guest running

So here "guest" refers to the same virtual machine, same disk file.

4. You wonder what would happen if a user tried to power on the guest on node1.

root@node2: virtsh start guest

5. It works :(

No warning, no error, no critical error. Just silent data corruption :(