Add support for an explicit RTC change event

Add support for an explicit  RTC change event

This introduces a new event type

   VIR_DOMAIN_EVENT_ID_RTC_CHANGE

This event includes the new UTC offset measured in seconds.
Thus there is a new callback definition for this event type

 typedef void (*virConnectDomainEventRTCChangeCallback)(virConnectPtr conn,
                                                        virDomainPtr dom,
                                                        long long utcoffset,
                                                        void *opaque);

If the guest XML configuration for the <clock> is set to
offset='variable', then the XML will automatically be
updated with the new UTC offset value. This ensures that
during migration/save/restore the new offset is preserved.

* daemon/remote.c: Dispatch RTC change events to client
* examples/domain-events/events-c/event-test.c: Watch for
  RTC change events
* include/libvirt/libvirt.h.in: Define new RTC change event ID
  and callback signature
* src/conf/domain_event.c, src/conf/domain_event.h,
  src/libvirt_private.syms: Extend API to handle RTC change events
* src/qemu/qemu_driver.c: Connect to the QEMU monitor event
  for RTC changes and emit a libvirt RTC change event
* src/remote/remote_driver.c: Receive and dispatch RTC change
  events to application
* src/remote/remote_protocol.x: Wire protocol definition for
  RTC change events
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h,
  src/qemu/qemu_monitor_json.c: Watch for RTC_CHANGE event
  from QEMU monitor