New virNetworkDef utility functions

New virNetworkDef utility functions

Later patches will add the possibility to define a network's netmask
as a prefix (0-32, or 0-128 in the case of IPv6). To make it easier to
deal with definition of both kinds (prefix or netmask), add two new
functions:

virNetworkDefNetmask: return a copy of the netmask into a
virSocketAddr. If no netmask was specified in the XML, create a
default netmask based on the network class of the virNetworkDef's IP
address.

virNetworkDefPrefix: return the netmask as numeric prefix (or the
default prefix for the network class of the virNetworkDef's IP
address, if no netmask was specified in the XML)