Use `--portcount` (>=4.3) or `--sataportcount` (<4.3) depending on VBox version.

Fixes dotzero/vagrant-debian-wheezy-64#15.
Merge pull request #16 from xurizaemon/issue-15

Fix issue #15 and #16 Use `--portcount` (>=4.3) or `--sataportcount` (<4.3) depending on VBox version.