iotests: Add missing 'blkdebug::' in 040

040 tries to invoke pause_drive() on a drive that does not use blkdebug.
Good idea, but let's use blkdebug to make it actually work.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20171109203025.27493-3-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>