fix: check for running VM processes using kill -0

[flexiondotorg]

Description

Change how VM processes are inspected to test if they are running but using kill -0 <pid>. Use kill -0 is portable between macOS and Linux and the BSD and GNU tools. quickemu also uses kill -0, which was introduced as part of the macOS porting work.

I've tested this on Linux to make sure there are no regressions; it needs validating on macOS.

• Fixes bug: [macos_desktop] active VMs are not detected #179

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have performed a self-review of my code
• I have tested my code in common scenarios and confirmed there are no regressions
• I have added comments to my code, particularly in hard-to-understand sections

[ymauray]

Looks good to me ! The "stop" button is active when the VM is running.