Fixes to get node-libvirt working on newer versions of node.js (v0.10.5)

Renamed the target to 'libvirt' and added 'product_prefix' to match the
definition in src/node_libvirt.cc's NODE_MODULE. This fixes a problem building
and using the module using newer versions of node (0.10.5). Also updated
the 'main' path in the package.json.