vsp change volume's get rule

[tanjiangyu-ghca]

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

Merging #599 (552cbcf) into vsp1130 (4ba8e59) will decrease coverage by 0.02%.
The diff coverage is 60.00%.

@@             Coverage Diff             @@
##           vsp1130     #599      +/-   ##
===========================================
- Coverage    71.37%   71.34%   -0.03%     
===========================================
  Files          187      187              
  Lines        23067    23071       +4     
  Branches      3508     3510       +2     
===========================================
- Hits         16463    16461       -2     
- Misses        5490     5492       +2     
- Partials      1114     1118       +4     

Impacted Files	Coverage Δ
delfin/drivers/hitachi/vsp/vsp_stor.py	68.85% <60.00%> (+0.29%)	⬆️
delfin/drivers/fake_storage/__init__.py	93.90% <0.00%> (-1.53%)	⬇️