vl: Make -machine $TYPE,help and -accel help print to stdout

Command line help help explicitly requested by the user should be
printed to stdout, not stderr.  We do elsewhere.  Adjust -machine
$TYPE,help and -accel help to match: use printf() instead of
error_printf().

Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Message-Id: <20190417190641.26814-10-armbru@redhat.com>