iotests: 109: Filter out "len" of failed jobs

Mirror calculates job len from current I/O progress:

    s->common.len = s->common.offset +
                    (cnt + s->sectors_in_flight) * BDRV_SECTOR_SIZE;

The final "len" of a failed mirror job in iotests 109 depends on the
subtle timing of the completion of read and write issued in the first
mirror iteration.  The second iteration may or may not have run when the
I/O error happens, resulting in non-deterministic output of the
BLOCK_JOB_COMPLETED event text.

Similar to what was done in a752e478, filter out the field to make the
test robust.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>