report: get value for VmCore only once

[bdrung]

Ease mypy to determine the type of self["VmCore"] by getting the value of VmCore only once and store it in a vm_core variable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.52%. Comparing base (3588162) to head (a9f436a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #611   +/-   ##
=======================================
  Coverage   84.52%   84.52%           
=======================================
  Files         104      104           
  Lines       21000    21001    +1     
  Branches     3219     3219           
=======================================
+ Hits        17751    17752    +1     
  Misses       2808     2808           
  Partials      441      441           

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