Better documentation for import-vbox option

[dani-sanchez-dorado]

Just document that the "import-vbox" option just convert the image files, does not create a virtual machine.
It's not declared explicity. It will improve the documentation.

And, just a comment: whould be possible to import vdf files? Are very similar to vdi files. Ova files are the same, but zip commpressed.

The trasnformation of files worked perfect. It's just I expected the full convert disk + virtual machine.

Thanks in advance!