iotests: Improve _filter_qemu_img_map

Although _filter_qemu_img_map documents that it scrubs offsets, it
was only doing so for human mode.  Of the existing tests using the
filter (97, 122, 150, 154, 176), two of them are affected, but it
does not hurt the validity of the tests to not require particular
mappings (another test, 66, uses offsets but intentionally does not
pass through _filter_qemu_img_map, because it checks that offsets
are unchanged before and after an operation).

Another justification for this patch is that it will allow a future
patch to utilize 'qemu-img map --output=json' to check the status of
preallocated zero clusters without regards to the mapping (since
the qcow2 mapping can be very sensitive to the chosen cluster size,
when preallocation is not in use).

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170507000552.20847-9-eblake@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
