My commit b8b42ca0 added support for formatting the vsock
command line without actually checking if it's supported.

Add it to the per-device validation function.

https://bugzilla.redhat.com/show_bug.cgi?id=1291851Reported-by: NJohn Ferlan <jferlan@redhat.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NJiri Denemark <jdenemar@redhat.com>