xhci: add property to turn on/off streams support

streams support in usb-redir and usb-host works only with recent enough
versions of the support libraries (libusbredir and libusbx).  Failure
mode is rather unelegant:  Any stream usb transfers will throw stall
errors.  Turning off support for streams in the xhci host controller
will work better as the guest can figure beforehand that streams are
not going to work.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>