Don't spam logs with "port 0 must be in range" errors

Don't spam logs with "port 0 must be in range" errors

Whenever virPortAllocatorRelease is called with port == 0, it complains
that the port is not in an allowed range, which is expectable as the
port was never allocated. Let's make virPortAllocatorRelease ignore 0
ports in a similar way free() ignores NULL pointers.