[Libvir] testing libvirtd: don't hard-code $HOME/.libvirt/libvirt-sock

[Jim Meyering]

I've written a test to exercise libvirtd a little.
Unfortunately, if I'm already running libvirtd, that test fails
and I can't do anything about it:

  $ ./libvirtd & ./libvirtd
  Failed to bind socket to '@/j/.libvirt/libvirt-sock': Address already in use
  [Exit 2]

Any objection to providing an option to libvirtd that specifies
that directory (also used to form server->logDir)?

It feels like it should come from the config file, but I haven't
looked at details yet.