iotests: improve 'not run' message for nbd-multiconn test

The test actually requires Python bindings to libnbd rather than libnbd
itself. Clarify that inside the message.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Hanna Reitz <hreitz@redhat.com>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20230906140917.559129-3-den@openvz.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

tests/qemu-iotests/tests/nbd-multiconn

@@ -142,4 +142,4 @@ if __name__ == '__main__':
 
         iotests.main(supported_fmts=['qcow2'])
     except ImportError:
-        iotests.notrun('libnbd not installed')
+        iotests.notrun('Python bindings to libnbd are not installed')