Make virtualbox-guest-additions to compile

Currently official build for guest additions will fail due to some
unset/wrong set variables. This is an workaround for it - added KERN_VER
variable which is set to current kernel version, so this ebuild cannot
be run against new kernel version while executed on older/other kernel
version.

app-emulation/virtualbox-guest-additions/files/vboxclient.desktop

@@ -0,0 +1,7 @@
+[Desktop Entry]
+Type=Application
+Name=VirtualBox Client Service
+Exec=VBoxClient-all
+Terminal=false
+X-KDE-StartupNotify=false
+StartupNotify=false