hw/qxl: qxl_send_events: nop if stopped

Added a trace point for easy logging.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=870972

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
