Restore functionality of 'is_open' field in portlist

[cmaglie]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• Tests for the changes have been added (for bug fixes / features)

• What kind of change does this PR introduce?
  Restore functionality of 'is_open' field in portlist

• What is the current behavior?
  is_open is always false

• What is the new behavior?
  is_open is true while the port is open for monitoring

• Does this PR introduce a breaking change?
  No

• Other information:

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 21.43%. Comparing base (8e28a0c) to head (ff59a8b).

Files	Patch %	Lines
serial.go	0.00%	17 Missing ⚠️
serialport.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #939      +/-   ##
==========================================
- Coverage   21.60%   21.43%   -0.18%     
==========================================
  Files          42       42              
  Lines        2495     2515      +20     
==========================================
  Hits          539      539              
- Misses       1859     1879      +20     
  Partials       97       97              

Flag	Coverage Δ
unit	21.43% <0.00%> (-0.18%)	⬇️

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[alessio-perugini]

Tested on linux ✔️