This:
than I could use a series of this function in order to read all of the column, save thoose that I want and discard the undesireble ones.
is conflicting with
But I don't want to read the entire line directly; i want to read just the "cpu" part. In this case, I would use the Qt function "readLine(3)" as a way of doing it
So either its ok for you to read the whole line, parse it, and discard what you don't need , or its not ok.
You have to decide.
If I'm not confusing things, when I began learning C++ in the university my professor show me a function, in C++, that one could use to read a line and stop reading when a empty space appear.
Various libs implement readline() function, which usually reads until the first \r\n, including Qt (see QDevice or QFile, QTextStream)

i want to read just the "cpu" part. In this case, I would use the Qt function "readLine(3)" as a way of doing it
This prerequisites that you know the correct starting points of your columns.
If this is the case you can jump directly to a start of a desired "column" by using seek().
Note, you will have to bookkeep your position in the file, as the file is serial, and "columns" are only a logical interpretation done by you.