Implement default hypervisor options on settings

[nesitor]

Problem: The unique way to set the default hypervisor to use is hard-coded on the models.py class, so we can't select another one by default.

Solution: Implement INSTANCE_DEFAULT_HYPERVISOR field on the orchestrator configuration variables.

[hoh]

One comment, the rest looks fine.

[hoh]

Since this changes the settings, it should go in setup() instead of check()

[github-actions]

The modifications include updates to the configuration settings of the VM execution engine in conf.py. This could potentially impact other parts of the system that rely on these configurations, such as the Aleph.im platform. The changes also involve updating the instance hypervisor setting in models.py.

The PR includes a conditional check for QEmu support in check() function which is used to validate if the required command exists or not. If it doesn't, an error message is raised indicating that the command isn't available. This could potentially cause issues on systems where QEmu isn't installed.

In conclusion, this PR requires a high level of expertise and attention as it involves changes to multiple files in different parts of the system. It also includes potential risks for introducing bugs or breaking existing functionality. Only experienced developers should review this PR.

The response ends with the category ('BLACK') followed by the explaining rationale, using markdown format and bullet points.

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[codecov]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 35.19%. Comparing base (3c56ddc) to head (0164b6a).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/aleph/vm/models.py	0.00%	3 Missing ⚠️
src/aleph/vm/conf.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #573      +/-   ##
==========================================
- Coverage   35.19%   35.19%   -0.01%     
==========================================
  Files          53       53              
  Lines        4844     4850       +6     
  Branches      572      574       +2     
==========================================
+ Hits         1705     1707       +2     
- Misses       3120     3124       +4     
  Partials       19       19              

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