virsh create somefile.xml creates my machine just fine but when I shut the machine down the whole thing disappears. Machines I made with the virt-manager GUI are persistent (stick around after shutdown) and the xml file is derived from those virt-manager created machines.
If you don't have the xml for the running vm (eg. after transient migration), you can do
The VM should now be listed. Or
There should be a line with 'Persistent: yes'.