vfs: add callback readv and writev handlers

[dcavalcante]

A mounted VFS causes callback readv and writev to call missing handlers, breaking both virtual and real file descriptors.

This adds both handlers and preserves I/O for real descriptors..

Fixes: #66532

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.40%. Comparing base (9746ebc) to head (7710c7a).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #66548   +/-   ##
=======================================
  Coverage   90.40%   90.40%           
=======================================
  Files         791      791           
  Lines      276120   276132   +12     
  Branches    53022    53039   +17     
=======================================
+ Hits       249618   249645   +27     
+ Misses      16897    16894    -3     
+ Partials     9605     9593   -12     

Files with missing lines	Coverage Δ
lib/internal/vfs/setup.js	87.26% <100.00%> (+0.13%)	⬆️

... and 20 files with indirect coverage changes

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