By using strncasecmp, we allow for arbitrary characters after the
"on"/"off" string. Fix this by switching to strcasecmp.

Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>