create virSocketAddrGetIpPrefix utility function

create virSocketAddrGetIpPrefix utility function

Create the utility function virSocketAddrGetIpPrefix() to
determine the prefix for this network.  The code in this
function was adapted from virNetworkIpDefPrefix().

Update virNetworkIpDefPrefix() in src/conf/network_conf.c
to use the new utility function.

Signed-off-by: Gene Czarcinski <gene@czarc.net>