feat(lxd): store and check PID when launching instances

[mr-cal]

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The PID of the process that creates an instance is stored in the instance's LXD config and is used when launching an instance.

The is_valid() check for base instances now ensures an instance is stopped and has been fully setup. If another process is busy launching and setting up the base instance, then is_valid() will wait (and eventually time out). If the process that launched the base instance is inactive, then the instance is auto-cleaned (deleted and recreated).

This last statement has a slight behavioral change. Previously, craft-providers raised a generic error (Instance setup failed. Check LXD logs for more details) and required the user to delete the base instance manually. Now, the bad base instance will be auto-cleaned. I think this is OK because this aligns with the design of the rest of launcher module where errors and misconfigured instances are auto-cleaned.

I recommend reviewing this per-commit.

The base instance has a new config value (the pid), so this must be paired with #465.

Fixes #448
Fixes #455
(CRAFT-2221)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bc946f0) 99.00% compared to head (40903d0) 99.01%.
Report is 1 commits behind head on main.

❗ Current head 40903d0 differs from pull request most recent head 70bf430. Consider uploading reports for the commit 70bf430 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #464   +/-   ##
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  Coverage   99.00%   99.01%           
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  Files          57       57           
  Lines        4532     4558   +26     
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+ Hits         4487     4513   +26     
  Misses         45       45           

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[lengau]

LGTM