Minor fix: Some hardware architecture don't support the --vtxux for VBoxManage modifyvm

My current hardware architecture has no support for [Intel VT-x](http://www.virtualbox.org/manual/ch08.html).

> --vtxux on|off: If hardware virtualization is enabled, for Intel VT-x only, this setting enables or disables the use of the unrestricted guest mode feature for executing your guest.

This hotfix makes the usage of `--vtxux` optional.