Change file permissions on instances

[Sploder12]

This PR changes file permissions on instance images and their cloud-init.iso files to be read/write only by root. Previously the files could be read by all.

MULTI-1403

[codecov]

Codecov Report

Attention: Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.

Project coverage is 88.86%. Comparing base (439285b) to head (b209ce1).
Report is 41 commits behind head on main.

Files with missing lines	Patch %	Lines
src/daemon/default_vm_image_vault.cpp	50.00%	2 Missing ⚠️
src/utils/vm_image_vault_utils.cpp	50.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3715   +/-   ##
=======================================
  Coverage   88.85%   88.86%           
=======================================
  Files         256      256           
  Lines       14557    14593   +36     
=======================================
+ Hits        12935    12968   +33     
- Misses       1622     1625    +3     

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[levkropp]

Looks good, it seems that codecov wants to see test coverage for some lines:

• if (!MP_PLATFORM.set_root_as_owner(path)) in write_to
• extract_image doesn't seem to have tests for it at all?

[Sploder12]

I have decided that the vm_image_vault tests are out of the scope of this PR. This is because writing tests would require a significant refactor of unrelated code (see 1653 in Jira).