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.
if you look at the latest version of the proxy code, it has been broken
down to allow multiple plugins (e.g outside of the initial proxy code).
this change conflict with yours (most code wasn't really changed, just
moved a bit), but will not allow a clean merge, in git terms, the way to
fix it is called a rebase :)