The virDomainBlockPull* family of commands are enabled by the
following HMP/QMP commands: 'block_stream', 'block_job_cancel',
 'info block-jobs' / 'query-block-jobs', and 'block_job_set_speed'.

* src/qemu/qemu_driver.c src/qemu/qemu_monitor_text.[ch]: implement disk
  streaming by using the proper qemu monitor commands.
* src/qemu/qemu_monitor_json.[ch]: implement commands using the qmp monitor