iotests: Change coding style of _filter_nbd in 083

iotests: Change coding style of _filter_nbd in 083

In order to be able to move _filter_nbd to common.filter in the next
patch, its coding style needs to be adapted to that of common.filter.
That means, we have to convert tabs to four spaces, adjust the alignment
of the last line (done with spaces already, assuming one tab equals
eight spaces), fix the line length of the comment, and add a line break
before the opening brace.

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>