fixes dhcp server virsh.rb to interpret prefix

The virsh.rb loadSubnets method was only looking for a definition of a netmask inside the libvirt network XML, despite netmask not being there in case a prefix attribute was defined. netmask and prefix are mutually exclusive by libvirt definition.

Also the ipaddr library has no easy way of retrieving a readable netmask from a prefix, albeit this functionality is provided by the inspect method, in this case the same thing was done as in inspect.