tools: move more libraries into tools/libs, cleanup libs building

Move some more libraries under tools/libs, including libxenctrl.

This series does a lot of 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 not to 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

PS: This is a test of the gitlab-centered workflow.