Add libusb dependency

Boxes is going to use libusb_device for filtering out keyboard and mouse.

https://bugzilla.gnome.org/show_bug.cgi?id=698430
Bump spice-gtk dependency

Boxes is going to use an API to access underlying libusb device that was
introduced in 0.27 release.

https://bugzilla.gnome.org/show_bug.cgi?id=698430
spice-display: Don't expose kbd & mouse

As a USB device is no longer available on the host after being redirected
to guest, do not expose keyboards and mice to the user.

https://bugzilla.gnome.org/show_bug.cgi?id=698430