change --sataportcount to --portcount

After setting my hard drive interface to “sata”, I hit a VBoxManage error during the build process: `Unknown option: --sataportcount`.  Looking at the docs for [VBoxManage storagectl][1], it appears that this option’s name is actually `--portcount` (as opposed to `--sataportcount`).

[1]: https://www.virtualbox.org/manual/ch08.html#vboxmanage-storagectl