original xhci code (the one which used libusb directly) used to use
'background transfers' for iso streams.  In upstream qemu the iso
stream buffering is handled by usb-host & usb-redir, so we will
never ever need this.  It has been left in as reference, but is dead
code anyway.  Rip it out.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>