1. Fire both property-pre-del:netvm and property-del:netvm - those
events should be fired in pairs - especially one may assume the other
will be called too. This is the case here - one disconnect old netvm,
the other connect the new one.
2. Remove spurious 'newvalue' argument for property-del:netvm event.
3. Fix logic for default_fw_netvm/default_netvm usage. The former is
used if vm.provides_network=True.