add chunksize to vchunk interface

[JacksonYao287]

when implementing fixed pg size, we need to know the exact size of a real chunk. add this interface to support this function.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.88%. Comparing base (1a0cef8) to head (e6d0ad4).
Report is 80 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/device/vchunk.cpp	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #572       +/-   ##
===========================================
+ Coverage   56.51%   66.88%   +10.37%     
===========================================
  Files         108      109        +1     
  Lines       10300    10585      +285     
  Branches     1402     1430       +28     
===========================================
+ Hits         5821     7080     +1259     
+ Misses       3894     2825     -1069     
- Partials      585      680       +95     

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[xiaoxichen]

can we targeting all fix_pg_size change to a branch and tested ?

[JacksonYao287]

can we targeting all fix_pg_size change to a branch and tested ?

ATM, actually, I am not quite sure how many changes will we exactly need for fix pg size. so I add this small step here, so that it will not block the initial development of fixed pg size feature.