iotests: Rename filter_nbd to _filter_nbd in 083

In the patch after the next, this function is moved to common.filter.
Therefore, its name should be preceded by an underscore to signify its
global availability.

To keep the code motion patch clean, we cannot rename it in the same
patch, so we need to choose some order of renaming vs. motion. It is
better to keep a supposedly global function used by only a single test
in that test than to keep a supposedly local function in a common* file
and use it from a test, so we should rename the function before moving
it.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>