add /etc/default/libvirt-bin generation for debian system.

This pull request allows puppet to generate the libvirt-bin default file used by the init script of libvirt daemon.
It's the equivalent of sysconfig for red hat ( I  guess ).
 
Example :
 
    class { 'libvirt':
      virtinst           => false,
      listen_tcp         => true,
      listen_tls         => false,
      auth_tcp           => "none",
      tcp_port           => 16510,
      unix_sock_group    => "libvirt",
      default            => {
        "start_libvirtd" => "yes",
        "libvirtd_opts"  => "-d -l"
      }    
  }

Matt