fix(hypercall): bound read buffer to VM memory

[Abhinavmadake]

read resolved the guest buffer with host_address + from_raw_parts_mut, so a guest-controlled buf/len could point outside the guest mapping. write, getdents and serialwrite already bound their buffers with slice_at/slice_at_mut; this makes read consistent.

• use slice_at_mut so an out-of-range or oversized buffer returns -EFAULT instead of reading or writing past VM memory
• add tests: valid read, buffer crossing the end, buffer past memory, oversized len

Fixes #1499

[Abhinavmadake]

thanks, done

[jounathaen]

Nice, thank you!

[codecov]

Codecov Report

❌ Patch coverage is 97.95918% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.93%. Comparing base (ff7ce4b) to head (22172d5).

Files with missing lines	Patch %	Lines
src/hypercall/by_fd.rs	97.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1500      +/-   ##
==========================================
+ Coverage   79.69%   79.93%   +0.23%     
==========================================
  Files          41       41              
  Lines        5640     5682      +42     
==========================================
+ Hits         4495     4542      +47     
+ Misses       1145     1140       -5     

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