qemu-options.hx: fix tls-channel help text

Remove the default compiled out tunnel channel, add the always available
cursor channel. Optimally the man page would depend on compiled in
options, but that's harder to do.

RHBZ: 688586

Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>