vl: load plugins as late as possible

There is no need to load plugins in the middle of default device processing,
move -plugin handling just before preconfig is entered.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>