util: rename/move iptablesFormatNetwork to virSocketAddrFormatWithPrefix

util: rename/move iptablesFormatNetwork to virSocketAddrFormatWithPrefix

This function formats an address + prefix as, e.g. 192.168.122.0/24,
which is useful in places other than iptables. Move it to
virsocketaddr.c and make it public so that others can use it. While
moving, the bit that masks off the host bits of the address is made
optional, so that the function is more generally useful.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>