The QEMU monitor command 'add_client' can be used to connect to
a VNC or SPICE graphics display. This allows for implementation
of the virDomainOpenGraphics API

* src/qemu/qemu_driver.c: Implement virDomainOpenGraphics
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h,
  src/qemu/qemu_monitor_json.c, src/qemu/qemu_monitor_json.h,
  src/qemu/qemu_monitor_text.c, src/qemu/qemu_monitor_text.h:
  Add binding for 'add_client' command