An option to choose between single quoted or double quoted attribute values could be desirable.

With single quote, qDebug() logging facilities do not escape the quoting. Sure we can use qDebug("%s", ...); but it's remove advantages of using the qDebug() << ...;

Also, double quoting make the task of copying output of QXmlStreamWriter to paste it in a .cpp file more tedious unless escaped, but if escaped, the logging output become tedious to read.

"<?xml version=\"1.0\" encoding=\"UTF-8\"?><stream:stream xmlns=\"jabber:client\" xmlns:stream=\"http://etherx.jabber.org/streams\" id=\"id\" from=\"ex.com\" to=\"foo@ex.com\" version=\"1.0\" xml:lang=\"en\">"

vs

"<?xml version='1.0' encoding='UTF-8'?><stream:stream xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams' id='id' from='ex.com' to='foo@ex.com' version='1.0' xml:lang='en'>"

Yeah it's estheticism but I like estheticism