It is not possible to perform an DNS SRV lookup without using the old Q3Dns classes.
Qt 3 used its own implementation of the DNS protocol and provided a low-level QDns class. Qt 4's QHostInfo class uses the system's gethostbyname() function from a thread instead.
The old QDns class has been renamed Q3Dns and moved to the Qt3Support library. The new QHostInfo class has a radically different API: It consists mainly of two static functions, one of which is blocking (QHostInfo::fromName()), the other non-blocking (QHostInfo::lookupHost()). See the QHostInfo class documentation for details.
So QHostInfo is not a replacement for looking up SRV records using Q3Dns, when you actually need the protocol implementation to look up an SRV record.
So it would be good if one could do this without requiring Qt3Support.