setup vbox profile location

[computezrmle]

Recently there were a lot of comments and suggestions regarding where to locate the VirtualBox configuration files.

See:
#6015
#6016
#6018

This PR is intended to combine them into a working solution on Linux/MacOS/Windows.

Major objectives:

1. use VirtualBox's default file locations where possible
2. use a well defined file location if (1.) is not possible

(1.) mitigates a possible race between VirtualBox Manager (launched by the user) and BOINC VMs (launched via VBoxHeadless).
Since the first VirtualBox process that starts up for a distinct user determines which vbox profile is used, all of them should use the same vbox profile.
(1.) also ensures that an interactive user can monitor the running VMs, e.g. on Windows.

(2.) must be used in environments where BOINC has not enough permissions to access the default vbox profile location, e.g. from inside MacOS's sandbox.

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

[Crystal-Pellet]

Tested the Windows version of this PR and concluded that VirtualBox is using again the default Oracle VirtualBox location for its xml- and log-files.
Unnecessary .VirtualBox and/or VirtualBox directories are not longer created in BOINC's datadirectory.

So OK to this version of vboxwrapper.

[davidpanderson]

Can you please also add comments (in vbox_common.h) for these fields:
string virtualbox_home_directory;
string virtualbox_scratch_directory;
string virtualbox_install_directory;
string virtualbox_guest_additions;

... saying what they're used for and how they're determined.

Also: I don't understand why these are elements of VBOX_BASE and not global vars.
(more generally I don't understand why we need VBOX_JOB, VBOX_VM, and VBOX_BASE at all.
But that's beyond the scope of this).

[computezrmle]

Can you please also add comments (in vbox_common.h) for these fields:
string virtualbox_home_directory;
string virtualbox_scratch_directory;
string virtualbox_install_directory;
string virtualbox_guest_additions;

As for virtualbox_scratch_directory
This is unused, hence I removed it.

Added comments for the others.
These variables have been introduced more than a decade ago together with comments explaining what they are used for.
Over the years the original comments obviously got lost.

VBOX_JOB, VBOX_VM, and VBOX_BASE are also used for more than a decade.
Changing this should indeed not be the scope of this PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 11.95%. Comparing base (ab9ae86) to head (4216226).
Report is 9 commits behind head on master.

Additional details and impacted files

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- Coverage     11.96%   11.95%   -0.01%     
  Complexity     1068     1068              
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  Lines         36940    36938       -2     
  Branches       8529     8529              
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- Hits           4419     4416       -3     
  Misses        32121    32121              
- Partials        400      401       +1     

see 1 file with indirect coverage changes