tools: move more libraries into tools/libs

Move some more libraries under tools/libs, including libxenctrl. This is resulting in a lot of cleanup work regarding building libs and restructuring of the tools directory.

I have (for now) left out some more libraries like libxenguest and libxl, but I can have a try moving those, too, if wanted.

Please note that patch 8 ("tools: move libxenctrl below tools/libs") needs the related mini-os and qemu-trad patches applied in order to not break the build:

https://lists.xen.org/archives/html/xen-devel/2020-07/msg00548.html

https://lists.xen.org/archives/html/xen-devel/2020-07/msg00617.html