emulators/virtualbox-ose: Put vbox/networks.conf in the right place

Since virtualbox-ose 6.1.26 a new networks.conf file has been
introduced and the software looks for it in /etc/vbox.

This commit introduces a patch to make virtualbox look for it in
PREFIX/etc/vbox.

To help users with configurations already using this file in /etc/vbox
notes to UPDATING, pkg-message and to the main virtualbox executable
script have been added.

Also adding some comments to the Makefile to note that QA warning
due to reinplace being run but not having changed any file are
expected due to the design of this port Makefile.

PR: 261522