This patch changes the network filtering code to use libvirt's existing
IPv4 and IPv6 address parsers/printers rather than my self-written ones.

I am introducing a new function in network.c that counts the number of
bits in a netmask and ensures that the given address is indeed a netmask,
return -1 on error or values of 0-32 for IPv4 addresses and 0-128 for
IPv6 addresses. I then based the function checking for valid netmask
on invoking this function.