Comment 4 for bug 1670652

Well, for one it is a bug for others a feature :-)

Many many people consider it a huge improvement that the mouse can seamlessly leave and enter the guest GUI (for quite a few releases I think).

The behavior to some extend depends on the device that is used and guest support.
So depending on how you usually control/start you VM qemu/libvirt/virt-manager/... you might find good hints how to change the device types in the following links (there mostly people want it enabled, but the guides can be followed in reverse).
http://unix.stackexchange.com/questions/107633/how-do-i-get-my-mouse-back-from-qemu-kvm
http://serverfault.com/questions/457603/any-way-to-release-focus-on-a-kvm-guest-in-virt-manager-without-having-to-click

If that doesn't help you, please let me know how you manage&start your guests so I can focus on that.