New virSocketAddr utility functions

New virSocketAddr utility functions

virSocketPrefixToNetmask: Given a 'prefix', which is the number of 1
bits in a netmask, fill in a virSocketAddr object with a netmask as an
IP address (IPv6 or IPv4).

virSocketAddrMask: Mask off the host bits in one virSocketAddr
according to the netmask in another virSocketAddr.

virSocketAddrMaskByPrefix, Mask off the host bits in a virSocketAddr
according to a prefix (number of 1 bits in netmask).

VIR_SOCKET_FAMILY: return the family of a virSocketAddr