[libvirt] Some maintenance back-ports

I wanted to back-port commit 4372a7845acbc6974f6027ef68e7dd3eeb47f425 to various maintenance branches due to the fact that it changes how migration behaves and that would fix it for future uses at leas. I found out some compilation problems (mainly because I switched to gcc 6.2.0). I wanted to make it easier for others (well, Cole) to make a maintenance release, so I back-ported some other stuff to make various maint branches compile. All of them were trivial build-fixers and there were few per each branch and I think it's pretty pointless to send each of them to the list (and I'm also lazy). But in case you're interested, feel free to check and complain if you hate what I did, just so I know for next time. Have a nice day, Martin