vnc: fix qemu crash when not configure vnc option

Add missing vnc options: to, ipv4, ipv6 and fix
qemu crash.

Reproducer:
$ x86_64-softmmu/qemu-system-x86_64
qemu-system-x86_64: Invalid parameter 'to'
Segmentation fault (core dumped)

BTW the patch fix the below bug:
https://bugs.launchpad.net/qemu/+bug/1414222

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Don Slutz <dslutz@verizon.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
