block/mirror: Always call block_job_sleep_ns() The mirror block job is trying to take a clever shortcut if delay_ns is 0 and skips block_job_sleep_ns() in that case. But that function must be called in every block job iteration, because otherwise it is for example impossible to pause the job. Signed-off-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>