Python/iotests: Add type hint for nbd module

Python/iotests: Add type hint for nbd module

The test bails gracefully if this module isn't installed, but linters
need a little help understanding that. It's enough to just declare the
type in this case.

(Fixes pylint complaining about use of an uninitialized variable because
it isn't wise enough to understand the notrun call is noreturn.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-id: 20231006195243.3131140-2-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>