Emit graphics events when a SPICE client connects/disconnects

Emit graphics events when a SPICE client connects/disconnects

Wire up the domain graphics event notifications for SPICE. Adapted
from a RHEL-only patch written by Dan Berrange that used custom
__com.redhat_SPICE events - equivalent events are now available in
upstream QEMU (including a SPICE_CONNECTED event, which was missing in
the __COM.redhat_SPICE version).

* src/qemu/qemu_monitor_json.c: Wire up SPICE graphics events