Otherwise logrotate barfs:

  error: error accessing /var/log/libvirt/uml: No such file or directory
  error: libvirtd:1 glob failed for /var/log/libvirt/uml/*.log
  error: found error in /var/log/libvirt/qemu/*.log /var/log/libvirt/uml/*.log /var/log/libvirt/lxc/*.log , skipping

* qemud/Makefile.am: always create /var/log/libvirt/{lxc,uml} when
  installing the logrotate conf; not ideal, but easier than making
  the logrotate conf depend on which drivers are enabled