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.
note that this code will only executed based on the condition at https://github.com/thkrmr/smart-proxy/blob/f70a52b9f070da37c522150e6d01caf764f19395/lib/proxy/dhcp/subnet.rb#L3