Follow-up to bug#1024671.

VBoxVNC.so is now installed but I don't see the VNC extension pack present in
VirtualBox Preferences -> Extensions. I believe it's because VBoxVNC.so was
installed as-is, but it should be installed as a part of .vbox-extpack file.

See how Arch does that
https://git.archlinux.org/svntogit/community.git/tree/trunk/virtualbox-ext-vnc.install?h=packages/virtualbox:

post_install() {
  VBoxManage extpack install
"/usr/share/virtualbox/extensions/VNC-${1%%-*}.vbox-extpack" \
    --accept-license="$(sha256sum
/usr/share/licenses/virtualbox-ext-vnc/LICENSE | head --bytes=64)" >/dev/null
}

pre_remove() {
  VBoxManage extpack uninstall 'VNC' >/dev/null
}

And then package the resulted VNC-*.vbox-extpack file.