cs_instance: fix AttributeError in get_details

[resmo]

Fixes #79
/cc @livegrenier

[codecov]

Codecov Report

Merging #80 (681b631) into master (d16c15f) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage   84.25%   84.22%   -0.04%     
==========================================
  Files          56       56              
  Lines        5597     5599       +2     
  Branches     1255     1256       +1     
==========================================
  Hits         4716     4716              
  Misses        445      445              
- Partials      436      438       +2     

Impacted Files	Coverage Δ
plugins/modules/cs_instance.py	71.20% <0.00%> (-0.33%)	⬇️
plugins/modules/cs_router.py	61.15% <0.00%> (-0.83%)	⬇️
plugins/module_utils/cloudstack.py	72.05% <0.00%> (+0.23%)	⬆️

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