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>