libvirtd service: Move mutable configs to /var

Modifies libvirt package to search for configs in /var/lib and changes
libvirtd service to copy the default configs to the new location.

This enables the user to change e.g. the networking configuration with
virsh or virt-manager and keep those settings.

pkgs/development/libraries/libvirt/default.nix

@@ -40,7 +40,7 @@ stdenv.mkDerivation rec {
 
   configureFlags = [
     "--localstatedir=/var"
-    "--sysconfdir=/etc"
+    "--sysconfdir=/var/lib"
     "--with-libpcap"
     "--with-vmware"
     "--with-vbox"
@@ -58,7 +58,7 @@ stdenv.mkDerivation rec {
 
   installFlags = [
     "localstatedir=$(TMPDIR)/var"
-    "sysconfdir=$(out)/etc"
+    "sysconfdir=$(out)/var/lib"
   ];
 
   postInstall = ''