Modify Used Capacity Unit Conversion

[luojiagen]

What this PR does / why we need it:

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Release note:

[codecov]

Codecov Report

❌ Patch coverage is 35.29412% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.85%. Comparing base (f2765e1) to head (e6e5321).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
delfin/drivers/dell_emc/vplex/vplex_stor.py	35.29%	9 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #985      +/-   ##
==========================================
+ Coverage   70.83%   70.85%   +0.02%     
==========================================
  Files         195      195              
  Lines       23987    23991       +4     
  Branches     3674     3675       +1     
==========================================
+ Hits        16991    16999       +8     
+ Misses       5831     5826       -5     
- Partials     1165     1166       +1     

Files with missing lines	Coverage Δ
delfin/drivers/dell_emc/vplex/vplex_stor.py	79.11% <35.29%> (-0.25%)	⬇️

... and 1 file with indirect coverage changes

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