virtio-ccw: fix range check for SET_VQ

VIRTIO_PCI_QUEUE_MAX is already too big; a malicious guest would be
able to trigger a write beyond the VirtQueue structure.

Cc: qemu-stable@nongnu.org
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>