Comment 15 for bug 962376

You don't need to rebuild the VM - a simple:

sed -i 's#bin/kvm#bin/kvm-spice#g' /etc/libvirt/qemu/*.xml

will do the trick, after which you need to stop all your guests and then redefine them with lines like this:

virsh define /etc/libvirt/qemu/foo.xml

... to redefine a guest named "foo", etc. Alternately, you could just reboot if you like.

Of course, you'll still have problems like vga video not working, needing to install spice guest tools in guests, etc. But that will fix the issue keeping the guest from starting at all.