Extended 'nixops check' to take into account the attachment status of EBS volumes.

+---------+--------+-----+-----------+----------+----------------+--------------+-------------------------------------------------------------------------------+
| Name    | Exists | Up  | Reachable | Disks OK | Load avg.      | Failed units | Notes                                                                         |
+---------+--------+-----+-----------+----------+----------------+--------------+-------------------------------------------------------------------------------+
| machine | Yes    | Yes | Yes       | No       | 0.00 0.01 0.05 |              | volume ‘vol-34baca6f’ on device ‘/dev/xvdf’ has unexpected state: ‘attaching’ |
+---------+--------+-----+-----------+----------+----------------+--------------+-------------------------------------------------------------------------------+