QM disconnection issues

[stavros11]

Fixes two small issues with QM:

1. instrument.connect() has a small bug (never sets self.is_connected) which causes instrument.disconnect() to not work properly and thus Quantum Machines are never closed. This causes the offsets (for example in flux) to stay on forever, potentially heating the fridge.
2. Unrelated to the above: a hot fix on duration sweeps of baked pulses. This is temporary (as you know, I am hoping to simplify these kinds of sweeps in 0.2), but it makes the two-qubit chevron routine run a bit better so it would be good to have for now.

[codecov]

Codecov Report

Attention: Patch coverage is 11.11111% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 66.45%. Comparing base (0bd68b0) to head (fc98cf2).

Files	Patch %	Lines
src/qibolab/instruments/qm/sweepers.py	14.28%	6 Missing ⚠️
src/qibolab/instruments/qm/controller.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #839      +/-   ##
==========================================
- Coverage   66.57%   66.45%   -0.13%     
==========================================
  Files          50       50              
  Lines        6079     6081       +2     
==========================================
- Hits         4047     4041       -6     
- Misses       2032     2040       +8     

Flag	Coverage Δ
unittests	66.45% <11.11%> (-0.13%)	⬇️

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[alecandido]

Nothing to amend to the code itself. I didn't test it (but I expect you did)

[stavros11]

Nothing to amend to the code itself. I didn't test it (but I expect you did)

In fact, I discovered the first issue when one of the controllers was connected to an ammeter in the lab, so it is certainly tested.