Fix FAILED_INCONSISTENT case in vg_backup_single - typeo on 'if' condition.
In vg_backup_single, we should error out if we vg_read_error(vg) and the
error code we received was anything other than FAILED_INCONSISTENT.
Original code contained an error because C operator precedence.
Note - this was part of the vg_read() so no WHATS_NEW entry neceesary.