[UDisksDeviceBackend] Port introspect to QXmlStreamReader
It's quicker than QDom.
There's other places where it does that, too, but this one has been by far the most severe in terms of time spent. Spends around 15ms on my machine on Dolphin startup in here.
I know there's this whole "port this to ObjectManager" situation but it's not going anywhere and I'm too scared of it, but I'm tired of seeing Solid plop up in hotspot wherever I go, so let's just have some low hanging fruit optimizations and not postpone them indefinitely until a mystical ObjectManager port will happen...