Add '--shared-dir' as required if '-o' is missing
If virtiofsd is run without any option an error is shown with required arguments. Since, we accept both '--shared-dir' and '-o source=' we cannot define '--shared-dir' as required.
Let's set it as required unless the '-o' option is provided. This is not perfect, since it will not show '--shared-dir' as required if any '-o' is provided, not only '-o source', but I think is enough because '-o' is deprecated, and it will be removed eventually.
Edited by German Maglione