Verified on 5.7.1.0, in case there is an 'In Use' volume, the attach volume is not available, only when there is another volume, you can see the available volume on instance attach volumes list
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://rhn.redhat.com/errata/RHSA-2017-0320.html