I have a problem just like this. I was able to get the VM up and running, logged in, but whenever I try to install something or even just wait awhile doing nothing, the VM will just close itself randomly. Iv'e tried your edit, but it still does the same thing.
Since I don't get any error messages from the apple side or from virt manager, I really don't know where to even start troubleshooting.
If anyone has any information on this, it would be much appreciated.