gnu: libvirt: Don't use bogus cgroup.

Should fix <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=36634>.

* gnu/packages/virtualization.scm (libvirt)[source]: Add patch.
* gnu/packages/patches/libvirt-remove-ATTRIBUTE_UNUSED.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.