I'm on fedora, i followed this tutorial : http://vfio.blogspot.de/2015/05/vfio-gpu-how-to-series-part-4-our-first.html
I used virt-manager to assign usb host device to the vm but it doesn't work on my system (was working on centos), apparently since the device is use by the system libvirt cannot use it? i got an alert.
I tried to modify the xml but the device was already there, if i try to add the line virsh tells me it couldn't find usb controller with index 1 required for device.
I'm on it for several days now, any help would be more than welcome. At this point even if the mouse and keyboard are always assign to the vm even if it is not running i don't care...
The problem on fedora was coming from selinux. What one has to do is to edit the file located at /etc/selinux/config and replace enforcing by permissive or disabled. After a reboot you can check the status of selinux with /usr/sbin/getenforce It should indicate whathever change you made