iotests: Test error handling policies with block-commit

This tests both read failure (from the top node) and write failure (to
the base node) for on-error=report/stop/ignore.

As block-commit actually starts two different types of block jobs
(mirror.c for committing the active later, commit.c for intermediate
layers), all tests are run for both cases.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200214200812.28180-8-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>