Improve VirtualBox descriptor parsing

[Schamper]

Fixes #70.

[codecov]

Codecov Report

❌ Patch coverage is 90.51724% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.59%. Comparing base (b19a789) to head (de2c7f5).

Files with missing lines	Patch %	Lines
dissect/hypervisor/descriptor/vbox.py	90.51%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
+ Coverage   78.01%   78.59%   +0.57%     
==========================================
  Files          26       26              
  Lines        2202     2313     +111     
==========================================
+ Hits         1718     1818     +100     
- Misses        484      495      +11     

Flag	Coverage Δ
unittests	78.59% <90.51%> (+0.57%)	⬆️

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