Release of libvirt-0.7.4

0.7.3 was broken

* configure.in docs/news.html.in: release of 0.7.4
* configure.in libvirt.spec.in: require netcf >= 0.1.4
* src/Makefile.am: node_device/node_device_udev.h was missing from
  NODE_DEVICE_DRIVER_UDEV_SOURCES breaking compilation on platforms with
  udev