Let's say you have already run some kvm virtual machines, and you just upgraded your virt-manager to a version that supports Qemu/kvm. Wouldn't it be nice that those machines could be monitored by
your virt-manager?

Of course it is,so here's how to do it:

Just start to install a new virtual machine with the disk file of your already existing qemu-kvm image. ( Here you can download some qemu-kvm images.)

And when install starts, just stop it. Next time you start that machine again you'll find your pre-existing machine. An other benefit of this is that you don't have to worry about the bridging stuff, it's all done for you.

For stopping the install:
killall qemu

Next time you open virt-manager, the new machine is there and when you run it, it is accessible from all your other guests and your host.