All handling of defaults (default_* variables) is inside vl.c,
move default_net there too, so we can more easily refactor that
code later.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>