Use RAII for VME profiler

Now that object creation is done in nested function calls we can
use an RAII-type profiler to trace it. This makes the profiling
much simpler and more robust.

Also, the stack of profiling data in the VME profiler has to match
the stack of completion callbacks in the VME, so the push and pop
operations are synchronized now.

Task-number: QTBUG-37978
Change-Id: I1bc5e0665b88e5b3772e48c8676cdda3fae59e1b
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>