VirtualBox 7 Compatability Changes

- Created a PowerShell script `virtualbox-wrapper.ps1` to wrap calls to vboxmanage and only exit once the virtual machine has been terminated.
- Updated launch.json to use the PowerShell launch type to launch `virtualbox-wrapper.ps1` with the machine name supplied as an argument.
- Updated `readme.md` to reflect these changes.

virtualbox-wrapper.ps1

@@ -0,0 +1,16 @@
+param (
+  $MachineName
+)
+
+VBoxManage.exe startvm $MachineName
+
+$running=$true
+while($running) {
+  Start-Sleep -Seconds 1
+  $status=(VBoxManage.exe list runningvms)
+  if($status) {
+    $running=$status.contains($MachineName)
+  } else {
+    $running=$false
+  }
+}