vhost: Fix host panic if ioctl called with wrong index

Missed a boundary value check in vhost_set_vring. The host panics if
idx == nvqs is used in ioctl commands in vhost_virtqueue_init.

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>