vnc: distiguish between ipv4/ipv6 omitted vs set to off

vnc: distiguish between ipv4/ipv6 omitted vs set to off

The VNC code for interpreting QemuOpts does not currently
distinguish between ipv4/ipv6 being omitted, and being
set to 'off', because historically the 'ipv4' and 'ipv6'
options were just flags which did not accept a value.

The upshot is that if someone runs

  $QEMU -vnc localhost:1,ipv6=off

QEMU still uses PF_UNSPEC and thus may still bind to IPv6,
when it should use PF_INET.

This is another instance of the problem previously fixed
for chardevs in

  commit b77e7c8e99f9ac726c4eaa2fc3461fd886017dc0
  Author: Paolo Bonzini <pbonzini@redhat.com>
  Date:   Mon Oct 12 15:35:16 2015 +0200

    qemu-sockets: fix conversion of ipv4/ipv6 JSON to QemuOpts

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1452518225-11751-6-git-send-email-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>