vl: Fix latent bug with -global and onboard devices

main() registers the user's -global only after we create the machine
object, i.e. too late for devices created in the machine's
.instance_init().

Fortunately, we know the bug is only latent: the commit before
previous fixed a bug that would've crashed any attempt to create a
device in an .instance_init().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190308131445.17502-4-armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>