feat: use quickemu --vm /path/to/vm.conf --kill to stop virtual machines

[flexiondotorg]

I confirm this feature has not been previously requested

• I have searched the issues and this feature has not previously been requested

Is your feature request related to a problem? Please describe.

Quickgui currently uses killall and the VM name as process identifier to stop/kill a running VM: https://github.com/quickemu-project/quickgui/blob/main/lib/src/pages/manager.dart#L344

QEMU for macOS is unable to set a virtual machine's process name and, therefore, cannot stop/kill a virtual machine in this fashion.

Quickemu 4.9.6 has a reliable --kill argument that uses the .pid file for the given VM to identify the virtual machine and kill/stop it.

Describe the solution you'd like

Once #169 is implemented, Quickgui should:

• On Linux, use quickemu --vm /path/to/vm.conf --kill if Quickemu 4.9.6 or newer is available.
  • And fallback to the killall mechanism if Quickemu 4.9.5 or older is found.
• On macOS, use quickemu --vm /path/to/vm.conf --kill if Quickemu 4.9.6 or newer is available.
  • Disable the stop button if Quickemu 4.9.5 or older is found.

Describe alternatives you've considered

Quickemu looks up the process ID from the .pid file for the given VM and implements a native Dart cross-platform mechanism to kill the virtual machine.

Additional context

None.