Diskbus config

It provides config option to override disk_bus configuration on box.xml.

User can set disk interface to vertio  for  box that is defined to use ide/sata as disk interface.
Or it can solve a reverse case.

vagrant-mutate generate kvm box that respect original OVF definition.
User can use it as is and also config disk interface with this.

After successful test, box can be create `vagrant package` with best disk interface.