Extend graphics event to include UNIX socket

Extend graphics event to include UNIX socket

Not all VNC/SPICE servers use a TCP socket for their connections.
It is possible to configure a UNIX socket server. The graphics
event must thus include a UNIX socket address type.

* include/libvirt/libvirt.h.in: Add UNIX socket address type
  for graphics event
* src/qemu/qemu_monitor_json.c: Add 'unix' string to address
  type enum