Remove hardcoding of 4.3.10 version

Allow use against any (latest) Virtualbox 4.3.x.
Also avoid littering of downloaded extension pack file

Signed-off-by: Esben Haabendal <esben@haabendal.dk>
Merge pull request #1 from esben/virtualbox-4.3.x

Remove hardcoding of 4.3.10 version