iotests: Add test for cancelling a mirror job

iotests: Add test for cancelling a mirror job

We already have an extensive mirror test (041) which does cover
cancelling a mirror job, especially after it has emitted the READY
event.  However, it does not check what exact events are emitted after
block-job-cancel is executed.  More importantly, it does not use
throttling to ensure that it covers the case of block-job-cancel before
READY.

It would be possible to add this case to 041, but considering it is
already our largest test file, it makes sense to create a new file for
these cases.

Signed-off-by: Max Reitz <[email protected]>
Message-id: 20180501220509[email protected]
Signed-off-by: Jeff Cody <[email protected]>