Description:
When connecting to a virtual machine with Spice remote access and Spice USB redirection, no USB devices can be redirected from the client system to the virtual machine, even though no error messages are produced.

Additional info:
* spice-gtk3 0.21-2
* No additional messages appear in dmesg or journalctl other that what is normal when an USB device is plugged in. No output is written to stderr by remote-viewer or Gnome Boxes.


Steps to reproduce:
* Connect to a virtual machine with Spice that has at least one USB Redirection device configured with remote-viewer or Gnome Boxes.
* Plug in or select and USB device to be forwarded.
* The USB device will be released from the client system (e.g. if you selected an USB mouse, you will no longer be able to move your pointer with it, although of course it still appears in lsusb).
* However, no new USB device will appear in the virtual machine (tested with a Windows 7 system that did not show the "New hardware detected" bubble and an Arch live CD where lsusb did not list the new device).
* After a downgrade to spice-gtk3 0.20-2, the USB device gets recognized properly by the virtual machine and may be used.