Issue description

To use the usb redirection features of virt-manager orvirt-viewer in NixOS, the user currently has to put spice-gtk into environment.SystemPackages, create a setuid wrapper, and activate a polkit action. This is neither obvious nor user-friendly, and currently undocumented except for some code comments.

Creating a little NixOS module to hide these steps behind a single config option will make this much easier to use.

I plan to implement this for 18.09 but anyone feel free to do it earlier...