Fix IPv6 connectivity

Summary:
IPv6 is supported by LibVNCServer but for compatibility reasons its
socket is backed by another file descriptor.

This commit adds tracking IPv6 listen socket in the same way we now
track IPv4 one.

Test Plan: Tested by connecting through ::1 and fe80:: addresses

Reviewers: #plasma, #kde_applications, alexeymin

Reviewed By: alexeymin

Subscribers: ltoscano, alexeymin, apol, aacid

Differential Revision: https://phabricator.kde.org/D13209