spice-display: Reorganize 'USB Devices' property

The current way the code is organized doesn't allow for another
section to be further added after the 'USB Devices' section.

Based on the folder sharing UI mockups, we will add another
'Folder Sharing' section after 'USB Devices', so we need
some code reorganizing.

https://bugzilla.gnome.org/show_bug.cgi?id=730259