virt-manager: add missing gsettings schema

Without this it'll complain and abort when clicking "Take Screenshot" or
"Browse Local" when creating a new VM and looking for an CD-ROM image to boot
from:

GLib-GIO-ERROR **: Settings schema 'org.gtk.Settings.FileChooser' is not installed

(cherry picked from commit cb7c920e2493040d7ac82656bf4382f6db87098c)