I have just had this problem and it took me a while to figure out how to fix it, when it really ought to be up there in lights, so here is my attempt to help anyone else having the same issue.
I just rebooted my remote server, which uses KVM to create a test bed Linux install, for the first time since I put the whole KVM kit-and-kaboodle on it. But when I tried to restart the KVM client I got an error message:
Error starting domain: internal error Network 'default' is not active.
So here is how to fix that:
sudo virsh net-start default
sudo virsh net-autostart default