[libvirt] ongoing code transitions section on the wiki

[Cole Robinson]

Hi all, I added a 'Ongoing code transitions' section to the
BiteSizedTasks page on the wiki:

https://wiki.libvirt.org/page/BiteSizedTasks#Ongoing_code_transitions

BiteSizedTasks might not be the best location for it, but I wanted to
start a list and that page already exists.

I added 3 sections:

* g_autofree/g_autoptr, with some example conversion commits

* GLib in general, listing APIs that can replace custom libvirt code.
Currently the list is only g_key_file for virkeyfile, g_user_*_dir for
virGetUser*Dir, some virFile APIs for g_path_* APIs.

* gnulib removal. I filled in a few examples of possible gnulib module
replacements, and listed all the others as 'Unsorted/replacments unclear'.

If you know any GLib replacements for existing code or gnulib modules,
please fill them in there. Or any other incomplete code conversions
internally

Thanks,
Cole