blockjob: Add block_job_yield()
This will unset busy flag and put coroutine to sleep, can be used to wait for QMP complete/cancel. Signed-off-by: NFam Zheng <famz@redhat.com> Reviewed-by: NEric Blake <eblake@redhat.com> Signed-off-by: NKevin Wolf <kwolf@redhat.com>
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