Improve virSocketAddrMask[ByPrefix] API

Improve virSocketAddrMask[ByPrefix] API

The original version of these functions would modify the address sent
in, meaning that the caller would usually need to copy the address
first. This change makes the original a const, and puts the resulting
masked address into a new arg (which could point to the same
virSocketAddr as the original, if the caller really wants to modify
it).

This also makes the API consistent with virSocketAddrBroadcast[ByPrefix].