iotests: Omit length/offset test in 040 and 041

As of a follow-up patch to this one, the length of a mirror block job
will no longer directly depend on the size of the block device;
therefore, drop these checks from this test. Instead, just check whether
the final offset equals the block job length.

As 041 uses the wait_until_completed function from iotests.py, the same
applies there as well which in turn affects tests 030, 055 and 056. On
the other hand, a block job's length does not have to be related to the
length of the image file in the first place, so that check was
questionable anyway.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1414159063-25977-7-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
