Asynchronous event for BlockJob completion

Asynchronous event for BlockJob completion

When an operation started by virDomainBlockPull completes (either with
success or with failure), raise an event to indicate the final status.
This API allow users to avoid polling on virDomainGetBlockJobInfo if
they would prefer to use an event mechanism.

* daemon/remote.c: Dispatch events to client
* include/libvirt/libvirt.h.in: Define event ID and callback signature
* src/conf/domain_event.c, src/conf/domain_event.h,
  src/libvirt_private.syms: Extend API to handle the new event
* src/qemu/qemu_driver.c: Connect to the QEMU monitor event
  for block_stream completion and emit a libvirt block pull event
* src/remote/remote_driver.c: Receive and dispatch events to application
* src/remote/remote_protocol.x: Wire protocol definition for the event
* src/remote_protocol-structs: structure definitions for protocol verification
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h,
  src/qemu/qemu_monitor_json.c: Watch for BLOCK_STREAM_COMPLETED event
  from QEMU monitor