qemu: Skip USB controller validation when model=none

qemu: Skip USB controller validation when model=none

This is not useful right now, because the function is simply
not called at all for model=none USB controllers, but that's
going to change in a moment, when we start calling the
function during validation instead of command line generation.
Making this change ahead of time means that we can simply
move the code verbatim later.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>