A kernel update from synaptic will not effect the virtualbox or its image. If you run updates and virtualbox script is installed it will uninstall virtual box but the image is in your home under .Virtualbox/harddrive. running the script from menu installs the latest version. If your using the synaptic version the script should not be installed and nothing should happen in an update. At any rate unless you delete an image it should be found in your home in the hidden .Virtualbox under harddisk. When setting up a new version just select it from the browse menu in virtualbox rather then making a new drive.
edit That is to say if you run updates and the script for virtualbox is installed and it is updated it will effect virtualbox by uninstalling the version in use - then run the script again and reuse the old image. If no update for the script then nothing happens. I keep the scrip uninstalled and reinstall and run it if I feel the need rather then chance having my virtualbox removed during updates. I do that myself later if a new version is out.
