spice: reduce refresh rate in native mode

Now that cursor updates are out of the way qxl needs the refresh timer
only when when running in vga mode, for dirty bitmap checking.  In
native qxl mode the guest will notify us, so we don't need to poll and
can use the idle interval (one refresh wakeup every few seconds).

Cc: Marc-André Lureau <marcandre.lureau@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
