spice: use bottom half instead of refresh timer for cursor updates

spice: use bottom half instead of refresh timer for cursor updates

Calling directly doesn't work due to the qxl-render code running in
spice server thread context.  Meanwhile bottom half scheduling is
thread-safe though, so we can use that to kick a cursor update in
main i/o thread context.

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