ARRAY_SIZE() is simple to use and removes the need to pre-define
the size of the command arrays.

Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>
Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>